I'm so sorry your husband was unfaithful. No one deserves that pain.
As far as forgiveness goes, that's totally on an individual basis and depends on your own feelings regarding infidelity. For me personally, infidelity is the end of the line. I cannot forgive that and there will never be an excuse good enough to make it okay. Infidelity is for cowards.
My experience with blackouts from being blackout drunk in my college days is that I remember bits and pieces of things that I've done, but I've never had sex and not remembered it. My husband is an alcoholic and has been blackout drunk around me and during those times has has his most regrettable behavior but he also remembers tiny bits of what happened. BUT being blackout drunk isn't an acceptable excuse for behaving badly in my opinion and that's what it is.