Anybody know if it would be acceptable/safe to surf nude at Black's?
Yes you can surf nude there I just don't know about the quality of the waves.
You can surf sans suits at Blacks as all have said. My brother did when we were there years ago, but that was just to say he had surfed nekkie at Blacks. The waves can be really good from what we were told. It's a popular surfing spot and the surfers and naturists get along well. Keep in mind though, that it is rather chilly water, even in the heat of summer and wetsuits, even the short ones were the norm and most surfers there are not naturist inclined.
This is a cute video of some chicas who made the trek down to Blacks via the hard way. There is a much easier way to get down there, but wear shoes, not sandals!