Yeah, this is too important for him to be turned aside easily, now that they are finally talking about it. And perhaps the fact that it is all so raw for him is something of an incentive to talk about it, to try to ensure that at least he's done everything he can to avoid Ianto dying needlessly (again) too.

He doesn't put much stock in the idea of his blood holding the secret to immortality, at least not without the Blessing getting in on the act, but there's enough there to have him wondering, and he nods thoughtfully at Ianto's suggestion. "Good idea. Blood is definitely his thing." His lips twitch. "He's always said I tasted good."

Ianto may not have asked, but Jack certainly has! Or, rather, he didn't need to ask because Spike told him, both in words and with his reactions.