9. ... b5?
Black was likely trying to relieve pressure on the f7 pawn by forcing White to retreat the bishop on c4. Moreover, White's retreat would cost a tempo, allowing Black to begin catching up in development.
Unfortunately for Black, Morphy instead found a way to continue his attack.
10. Nxb5!
A nice sacrifice that allows White to continuously keep the Black king under attack. The next diagram shows the results of this sacrifice.
