I have exactly the same problem after going through the pain of levelling all the Stealth modules. It's glitched (as is Form Tetsudo! - the Armour variant)
The issue is that when the skill assesment is unlocked it fails to take into account any progress you have already made towards level 12 and starts counting from where you are. If you're at 7/11 like me, then that means you were at level 5 Stealth when you unlocked the assesment.
Luckilly I had not used Armour at all at that point, so have managed to get my Armour one done and am the only person i've encountered in 750 games with the Armour skill assesment tag! Woo! (would have been very frustrating otherwise, especially after getting 150 Air Stomps and Detonation Delays...) But i'm annoyed as the Stealth one is really the one I want, and as you say, Blind Spot is a real pain to level.