I also don’t remember anything specific about Harry bleeding in any chapter, but an opportunity to take it unawares would have been just before chapter 60, when Harry was sleeping in Quirrel’s presence.
A potential problem with Quirrel doing this is that the ritual’s requirements seem to distinguish between “forcibly” and “unknowingly”. It’s possible that he’ll have to do it by directly forcing Harry to give up his blood, not by deceiving or tricking him, or even letting him lie unconscious while he’s pulling it out.
I also don’t remember anything specific about Harry bleeding in any chapter, but an opportunity to take it unawares would have been just before chapter 60, when Harry was sleeping in Quirrel’s presence.
A potential problem with Quirrel doing this is that the ritual’s requirements seem to distinguish between “forcibly” and “unknowingly”. It’s possible that he’ll have to do it by directly forcing Harry to give up his blood, not by deceiving or tricking him, or even letting him lie unconscious while he’s pulling it out.