I've post before about Shiera Seastar and how I think she could have some type of effect on the story. Looking at her appearance and ties with Bloodraven, made me think of a quote from the Mystery Knight and Bloodraven's "glamour". So lets take a look.

Could Shiera's appearance be a glamour using her necklace?

Shiera's Appearance

Shiera was born with one dark blue eye and one bright green one, but the singers said that this flaw only accentuated her loveliness. She was the greatest beauty of her age, a slender and elegant woman, slim of waist and full of breast. She had the silver-gold hair of the Targaryens, thick and curling, and wore it very long. At some points in her life it fell well below her waist, almost to the back of her knees. She had a heart-shaped face, full lips, and her mismatched eyes were strangely large and full of mischief; her rivals said she used them to melt men's hearts.

and:

she was fond of ivory and lace and cloth-of-silver (but not gold, which she considered too vulgar). Her favorite piece of jewelry was a heavy silver necklace of emeralds and star sapphires, alternating. -SSM, Shiera Seastar: 7 March 2006

So we see Shiera's eyes seem to match her necklace (alternating emeralds/sapphires).

Which if we compare to Bloodraven's likely glamour as Maynard Plumm:

Plumm's cloak was as purple as name, though frayed about the edges and badly dyed. A moonstone brooch big as a hen's egg fastened it at the shoulder. Elsewise he wore dun-colored roughspun and stained brown leather.

and:

Dunk whirled. Through the rain, all he could make out was a hooded shape and a single pale white eye. It was only when the man came forward that the shadowed face beneath the cowl took on the familiar features of Ser Maynard Plumm, the pale eye no more than the moonstone brooch that pinned his cloak at the shoulder. -The Mystery Knight

They seem somewhat similar. Especially since they practiced the dark arts together:

Bloodraven proved to be a capable Hand, but also a master of whisperers who rivaled Lady Misery, and there were those who thought he and his half sister and paramour, Shiera Seastar, used sorcery to ferret out secrets. It became common to refer to his "thousand eyes and one," and men both high and low began to distrust their neighbor for fear of their being a spy in Bloodraven's employ. -TWOIAF, The Targaryen Kings: Aerys I

We also get another somewhat similar example in what Mel does with Rattleshirt/Mance:

"The bones help," said Melisandre. "The bones remember. The strongest glamors are built of such things. A dead man's boots, a hank of hair, a bag of fingerbones. With whispered words and prayer, a man's shadow can be drawn forth from such and draped about another like a cloak. The wearer's essence does not change, only his seeming." -ADWD, Melisandre I

Counterpoints:

• While characters like Mel, Alys Rivers and Shiera's mother are thought to be much older than they actually are, we know Shiera is the daughter of Aegon IV and we get her appearance as known from birth so unless the glamour just makes her irresistible or more beautiful, I don't know what else the glamour would be for since she already looks like that
• The comparison to Bloodraven's appearance change isn't really 1 to 1. As the moonstone brooch looks more like Bloodraven's original appearance

TLDR: A somewhat interesting parallel between Shiera Seastar's mismatched eyes and necklace as compared to glamour usage, but nothing conclusive.