I usually say, "Oh, what was the word? It was right there (on the tip of my tongue)..." or the third option, "I have it in my head but I can't think of it right now."
All of the options seem fine to me aside from a few errors.
-I can't remember it
-I can't think of it off the top of my head (this seems better to me than "I can't think of it offhand", that doesn't sound natural to me)
-I have it in my head but I can’t think of it right now
-It went blank (as Marie said, this is not used very often, I've never heard it before)
-I can’t think of it, it was in my mind a minute ago
-I got nothing
-I'm trying to remember it but I'm drawing a blank.