Tobiko is the most common English spelling for a sushi term referring to flying fish eggs/roe. Technically, the proper Japanese term for tobiko is tobiuonoko
(飛び tobi = fly/flying,
魚 uo = fish,
の子 no ko = children thereof).
Recipes and English labeling might also display this as tobikko or even tobico. Tobiko itself has a subtle, slightly salty flavor. Each egg is larger than a poppy seed but smaller than a sesame seed, therefore the texture has a soft grain and a tiny pop.
Tobiko comes in a rainbow of colors:
- Red / orange:
Food coloring is used to achieve this color, it is vibrant and translucent. This what you usually see on the outside or your (higher quality) california roll. This form is pretty easy to find at around $3 for a one oz. jar.
This is often referred to as ginger tobiko. I believe the color is mnemonic and artificial. Tobiko without any coloring are mostly clear with a yellow tinge like a miniture capsule of cod liver oil.
Flavoring the tobiko with wasabi makes them bright green and very spicy. This is the most expensive type of tobiko at $4 for a 1 oz. jar or $65 by the kilogram.
- Blue / purple:
Technically there's no reason this couldn't exist, but I have never seen it jarred for sale. Maybe one time Morimoto considered coloring his own tobiko in blue for a special artistic maki.
The novelty of black tobiko is that it looks like other, more expensive, kinds of caviar. However, black tobiko tastes just the same as common orange tobiko and although it's harder to find it isn't any more expensive.
On an interesting side note, if you try to look up 'tobiko' in your Japanese dictionary you won't find it. What you will find is tobikomijisatsu, committing suicide by throwing oneself in front of an onrushing train.