A shadow drake has an unusual innate power. In darkness or heavy shadows, it seems to merge with the shadows, becoming invisible even to ultravision and infra-vision. This drake enjoys stealth and is almost noiseless in flight, surprising foes five in six times if the area is in shadow or darkness. This drake can attack from shadows or darkness without becoming visible to prey.
All shadow drakes have the innate ability to create darkness twice per day as per the illusionist spell, except that such darkness lasts for 24 hours. They can also create a smaller area of darkness, as per the spell except that it lasts only five minutes and is 10’ across; this power may be used at will. If cast upon a creature that does not make its saving throw vs. spells, this spell blinds the victim for the spell’s duration.
Once a shadow drake has access to an area of magical darkness (even self-created), it can enter that darkness, wherein it can utilize other innate abilities. The first ability is to create a shadow duplicate of itself. This power is like the illusionist spell shades, except that the type of shadow monster created is an illusionary duplicate of the shadow drake without magical ability. This shadow duplicate cannot be told from the original, as per the illusionist spell mirror image. This power can be used but once per day. The second ability is to duplicate the effects of the illusionist spell demi-shadow monsters. There are no restrictions to this spell-like ability, and any creature the shadow drake can imagine can be created. This spell is always cleverly used to create a monster to cause the most disruption possible. This power may be used twice per day. The third ability available to shadow drakes while in magical darkness is rope trick, as per the illusionist spell. The shadow drake can hide in this extradimensional space for the duration of the spell, even if the magical darkness is dispelled. This power can be used twice per day.
Ancient shadow drakes gain the power to shift to the Astral plane and back once per month. They can take no other shadow drakes with them.
Shadow drakes of very old and ancient ages are known to have one further power, that of being able to dimension door from one area of magical shadow to another within a one-mile radius.
This sounds essentially like the shadow dragon's shadow blend ability.
A unique Su ability?
I'd recommend simplifying this to just assigning these as SLAs at the given age categories.
Akin to the crystal drake's specialized plane shift?
This sounds like a variation on the shadowdancer's shadow jump ability.
It also doesn't have a breath weapon, which maybe isn't such a big deal for such small dragons as drakes, but it's something nonetheless. The extra spellcasting could help make up for that. Or we could improve their sharp claws, like the fang dragon.
Yep, feels like shadow blend to me.A shadow drake has an unusual innate power. In darkness or heavy shadows, it seems to merge with the shadows, becoming invisible even to ultravision and infra-vision.
Make it stronger than the black dragon's darkness and different from the shadow dragon's create shadows (this isn't solid black right)? What about deeper darkness?All shadow drakes have the innate ability to create darkness twice per day as per the illusionist spell, except that such darkness lasts for 24 hours.
Let's tweak project image, which is fittingly an illusion (shadow) spell.Once a shadow drake has access to an area of magical darkness (even self-created), it can enter that darkness, wherein it can utilize other innate abilities. The first ability is to create a shadow duplicate of itself.
Shadow conjuration but limited to creatures?The second ability is to duplicate the effects of the illusionist spell demi-shadow monsters.
Similar enough to the 3.5e version of the spell?The third ability available to shadow drakes while in magical darkness is rope trick, as per the illusionist spell.
I'm surprised it can plane shift to the Astral instead of Shadow, but maybe because Shadow was only a demi-plane in 2e.Ancient shadow drakes gain the power to shift to the Astral plane and back once per month.
Reminiscent as well of the shadow dragon's dimension door and shadow walk.Shadow drakes of very old and ancient ages are known to have one further power, that of being able to dimension door from one area of magical shadow to another within a one-mile radius.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.