Roller delayed blowback firearms are most definitely not the same as a simple blowback design like an UZI, Colt SMG, etc.
So I don't think it's completely correct to say that the MP5 is a "blowback" design. HK has always described it as delayed roller locking blowback system (DRLBS) which to me distinguishes it from a basic blowback design.
I was thinking along SMG lines. Thanks for those examples.