It is normal that if you chose a situation where blur applies no bonus, dance of blade is better...
that's a non-argument.
and if you are fighting ships with ships, airI is better than dance of blades(that applies only to living units)...
but do you know many stacks without at least 1 unit with drill promotion ? or a stack without any archers ?
NeoMega : you are supposing :
1) you have access to many catapults/fireballs, enough to raze the city defense in 1turn.
or you have a sea access to the city. (blur+shadowalk needs at most 1mage+ 1adept, while 4-6fireballs needs 4-6 mages... much more economical.)
2) the ennemy city has lot of cultural defense ; if there are more building defense than CD, 1"shadowwalk" has more effect than 1 fireball or 1 cat.
3) you are at war with the guy.(a shadow, in friendly territory, with shadowalk is.... wow, incredible !, any defense due to archer range, walls, palisade, dungeon, barracks.. it is a LOT of +10%, +20%)
and then, even if you took out 80% of the defense, leaving 20%, half being building defense, 10% defense less is still nice. It's maybe not worth a lvl2 spell, but it is still nice.
In the end I think blur is OP (especially as a lvl 1spell).
shadowwalk is either worth a low quality lvl2 spell, or worth a normal lvl1 spell, but is counted as lvl2.
Shadowwalk combined with blur in 1spell is OP, but because blur is OP, not because of shadowwlak, but because of blur.