Hiya lemon4ik, you could use it(I WOULD'NT) but I've seen people use em...can pick em up in Poundland / Poundworld...Would be better if you could get some proper Mylar Though..will last you longer, more reflective and less chance of Hot-Spots..Plus the proper Mylar(Thicker & Backed) will be 100% Lightproof...don't think those Emergo Blankets are..neither is the cheap Panda Film on FleaBay> I know cuz I bought some(Never Again) depends on your Budget and where you Live..but Premade Grow Tents go for like £50 on FleaBay(for Small1)...but IF you've just gotta build(I Did ~ Time & Time again) then just look for a better Quality of Mylar...pays for itself in the end with less hassle....IMHO
Peace
EDIT: You could also just Paint Grow Box Matt White...I did and lots of peeps do too....