English grammar

Both are correct.

Perhaps "when he punched me" would be even better.

There is no need for the past perfect in your original sentence.

Anonymous Which is the correct sentence:"I blacked out after he had punched me"I blacked out after he punched me"

"Blacked out" is odd. Passed out would be more natural http://dictionary.cambridge.org/dictionary/british/pass-out

"Blacked out" is very natural in American English.

It's fine in BrE, too: http://dictionary.cambridge.org/dictionary/british/black-out

Anonymous"I blacked out after he had punched me"

I kind of agreed with Anon that "blacked out" in this context is not quite appropricate. If the context was " He kept drinking the whole night until he blacked out", it would have been correct. But he was knocked out which meant he was unconscious. So the sentence woukd have been much better written as : A wa

Anonymous So the sentence woukd have been much better written as : A was unconscious after B punched him out.

I can't agree. You have a register conflict between 'unconscious' and 'to punch out' and you've changed an action (blacked out) to a state (was).

CJ