frankenstein dracula variation 10...b5?!

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ruben72d

In the ending positon above and similar positions, arising from the frankenstein-dracula variation, I've always wondered why black is playing 10...b6 to capture the knight and not 10...b5. both moves make room for the bishop on b7 but after 10...b6 white has the possibility to take on b6 while with 10...b5 he has not. This results in black being a pawn up on the queenside without losing his normal kingside prospects. As it seems to be never played, there must be something wrong with it, although  It is not clear to me how b5 can turn out to be a wrong move by black.

ruben72d

nvm Qc5 seems to be the problem...

Kretinovich

b5 is much weaker i actually have a book on it.