I have one. Lessee...
I think it could work. It's noticeably shorter than the original arm by about 4mm, but it's about 30% wider at the muzzle, and so still looks good. Moreover, the tubelike projection under the RAC is perfect for one of the ERMLs, requiring only that you kitbash a second laser on the other side. I don't know how big the Cygnus's ammo feed is, but that piece might work here too.
A word of warning. Those weird tubes sticking out the Blade's back? Those are a true pain to put in. Not difficult, but really annoying. Aside from that, It's a really easy mini to work with. If my RAC arm weren't already slated for a Schwerer Gustav attempt, I'd try to upgrade my Blade with it.
Never bothered with the Blade, between no paint schemes for the Republic and the fact that the original configuration left me "Meh" compared tot he Legionairre, I just never bothered picking one up.
Only used it once so far in combat, but I like it, a good strike 'mech. The only weakness it has is having the armor of the light 'mech it is, but packing the firepower of a medium. You CANNOT let that firepower go to your head, or you'll get the 'mech shot out from under you.
[Edit] Here's a couple quick and dirty photos to show the size comparison.