As of patch R101 the ‘strongest color’ of AP stolen by Blade’s Nightstalker can get stuck on whatever color is the enemy team’s strongest color at the start of the match.
It doesn’t change when enemy team members are downed.
Also, if the enemy no longer has any AP of that particular color left, Nightstalker does not fall through to the 2nd strongest color.
(I believe it did used to do that before R101.)