repexp to collapse multiple blank lines into one

[B. T. Raven]

Is there a way to use M-C-% to flush extra blank lines from a buffer.
Something like:
Query replace regexp:  ^$+ [which of course doesn't work, presumably
because $ has to be last]
with:      C-j

I know about C-x C-o but that's interactive rather than batch mode.

Thanks,

Ed