The definition of blackout drunk is not remembering what happened. This /could/ simply be a failure to form lasting memories, rather than a loss of 'self-ness.' If we imagine a switch which put a person's long-term memory in read-only mode, would flipping that switch necessarily remove consciousness?
Consider also that brains have different mechanisms for short and longterm memory; functionally, I think we're typically using read-only long-term memory combined with read-write short-term memory, and then writing distillations into long-term memory as-needed. Disabling long-term memory writes in this model may be functionally indistinguishable from a normal operation, at least on shorter time scales.
(For my own part, when I was younger I occasionally got extreeeemely drunk, and would actually flip into some hyper-vigilance around whether I would remember things later; I remember a number of these 'will I remember this later?' moments, though I can't say how many I forgot!)
