You can also infuse Legendaries with higher light-level RARE weapons (blues) to raise their ATK/DEF value. Another Legendary is not required; blues give the same boost.

No, it's not a troll. No, I didn't believe it myself at first.

• The boost amount is the same.
• The infusion cost is the same.
• I infused a 250 Sidearm with a 275 blue shotgun and it raised the attack value to 270.
• This works with weapons, armor (including class armor), ghosts, etc.

I really regret deleting so many higher level blues now... >:[