Bloodraven & Dark Sister

Primarily due to Aegon IV (Unworthy)'s gift of Blackfyre to Daemon (the King who bore the Sword), the passing down of the Targaryen ancestral swords has always interesting. We know "why" Aegon gave Daemon the sword, but I thought it would be fun to look into how Bloodraven ended up with Dark Sister.

Fun fact: Neither Blackfyre nor Dark Sister have been mentioned in the main series yet.

Background

While Blackfyre was the sword possessed by every Targaryen king since the Conqueror, only two (Maegor and Jaehaerys I) also possessed Dark Sister.

The last known person to possess it before Bloodraven = Prince Aemon (The Dragonknight)

"Daeron . . ." Ser Eustace almost slurred the word, and Dunk realized he was half drunk. "Daeron was spindly and round of shoulder, with a little belly that wobbled when he walked. Daemon stood straight and proud, and his stomach was flat and hard as an oaken shield. And he could fight . With ax or lance or flail, he was as good as any knight I ever saw, but with the sword he was the Warrior himself. When Prince Daemon had Blackfyre in his hand, there was not a man to equal him . . . not Ulrick Dayne with Dawn, no, nor even the Dragonknight with Dark Sister. -The Sworn Sword

and:

In 136 AC, Aemon followed. He was as robust as Aegon as an infant, and as beautiful to look upon, but his brother's faults were not in him. He proved the greatest jouster and swordsman of his age—a knight worthy to bear Dark Sister. He became known as the Dragonknight for the three-headed dragon crest wrought in white gold upon his helm. To this very day some call him the noblest knight who ever lived and one of the most storied names to ever serve in the Kingsguard. -TWOIAF, The Targaryen Kings: Viserys II

Prince Aemon died sometime between 178 and 183AC:

However, Bethany found his royal embraces distressing. For comfort, she turned to a knight of the Kingsguard, Ser Terrence Toyne. The pair was discovered abed by Aegon himself in 178. Ser Terrence was tortured to death and both Lady Bethany and her father were executed. When Ser Terrence's brothers sought to avenge his death, Prince Aemon the Dragonknight was slain while defending his brother, King Aegon -TWOIAF, The Targaryen Kings: Aegon IV

Bloodraven was born in 175 AC, so I doubt that Aegon IV gave it to Bloodraven (as it would have surpassed Daemon and been memorable).

Bloodraven's Sword

We know that Dunk saw Bloodraven with it in ~205 AC. in King's Landing and he already had the sword:

Six years ago in King's Landing, Dunk had seen him with his own two eyes, as he rode a pale horse up the Street of Steel with fifty Raven's Teeth behind him. That was before King Aerys had ascended to the Iron Throne and made him the Hand, but even so he cut a striking figure, garbed in smoke and scarlet with Dark Sister on his hip. His pallid skin and bone-white hair made him look a living corpse. Across his cheek and chin spread a wine-stain birthmark that was supposed to resemble a red raven, though Dunk only saw an odd-shaped blotch of discolored skin. He stared so hard that Bloodraven felt it. The king's sorcerer had turned to study him as he went by. He had one eye, and that one red. The other was an empty socket, the gift Bittersteel had given him upon the Redgrass Field. Yet it seemed to Dunk that both eyes had looked right through his skin, down to his very soul. -The Sworn Sword

If we assume (as I did above) that Bloodraven was at least 13 when he got the sword that means he received the sword sometime between: 188 and 205 AC.

It should be noted that although he has this sword and is a good swordsman, he prefers to use a weirwood bow:

Although he bears the Valyrian blade Dark Sister than once belonged to Aegon's Visenya, his favorite weapon is a tall bone-white weirwood longbow. He is an expert bowman. His longbow is weirwood, and should be white.

Events that may have led to Bloodraven getting it

• Informing on the upcoming First Blackfyre Rebellion

In the end, years of such talk bore their fruit, and Daemon Blackfyre made his decision. Yet it was a decision he made rashly, for word soon reached King Daeron that Blackfyre meant to declare himself king within the turn of the moon. (We do not know how word came to Daeron, though Merion's unfinished The Red Dragon and the Black suggests that another of the Great Bastards, Brynden Rivers, was involved.) -TWOIAF, The Targaryen Kings: Daeron II

• Actions during the First Blackfyre Rebellion

The rebellion ended at the Redgrass Field, nigh on a year later. Some have written of the boldness of the men who fought with Daemon, and others of their treason. But for all their valor in the field and their enmity against Daeron, theirs was a lost cause. Daemon and his eldest sons, Aegon and Aemon, were brought down beneath the withering fall of arrows sent by Brynden Rivers and his private guards, the Raven's Teeth. This was followed by Bittersteel's mad charge, with Blackfyre in his hand, as he attempted to rally Daemon's forces. Meeting with Bloodraven in the midst of the charge, a mighty duel ensued, which left Bloodraven blinded in one eye and sent Bittersteel fleeing.

• Bloodraven/Missy Blackwood's Status at Court

Probably the least likely (but I needed a third). Missy was well liked even by Daeron:

(MISSY) BLACKWOOD: The best loved of the king's mistresses. Both younger and prettier than Lady Barba (albeit far less buxom), as well as more modest, Missy had a kind heart and generous nature that led even Queen Naerys herself—as well as the Dragonknight and Prince Daeron—to befriend her. During the five years of her "reign," Missy bore the king three bastards, most notably the boy Brynden Rivers (born 175), later called Bloodraven. Children by Melissa Blackwood: Mya, Gwenys, Brynden (Bloodraven).

Poor Missy/Naerys/Daemon/Daeron. I can't imagine being put in this position by the Unworthy.

Perhaps at first, Daemon Blackfyre merely indulged such talk for the sake of his vanity. After all, years had passed between the first men approaching Daemon and the actual rebellion. What, then, tipped Daemon over into proclaiming for the throne? It seems likely it was another of the Great Bastards: Ser Aegor Rivers, called Bittersteel. Perhaps it was his Bracken blood that made Aegor so choleric and so quick to take offense. Perhaps it was the ignominious fall of the Brackens in King Aegon's esteem, leading to his exile from Aegon's court. Or perhaps it was only his rivalry with his half brother and fellow bastard Brynden Rivers, who had been able to maintain his close relations at court—for Bloodraven's mother had been well loved during her life, and was fondly remembered, so the Blackwoods did not suffer as the Brackens did when the king cast off his respective mistresses. -TWOIAF, The Targaryen Kings: Daeron II

I do tend to think that Bloodraven already had Dark Sister when they dueled. If interested: The Duels Between Bittersteel & Bloodraven & Dark Sister and Blackfyre: Could they meet again?

Its also weird that Bloodraven was allowed to take Dark Sister to the Wall with him when Blackfyre was already out of Targaryen possession.

TLDR: With the Unworthy granting the sword Blackfyre to Daemon, it is a bit weird that Bloodraven ended up with the other Targaryen ancestral sword. I listed out a couple options on how it could have happened. My most likely bet is for informing on the First Blackfyre Rebellion.