Not just any black spot either. A black spot of random size larger than what you want to redact. That way you avoid leaking the size of what's being redacted. The size of what's being redacted can sometimes provide enough information to determine plausible contents: http://blog.nuclearsecrecy.com/2014/07/11/smeared-richard-fe...
Ok, but just to be clear, we're redacting faces here. There isn't much meaningful here other than an exceptionally rough indication of age/development.
The examples on the Signal website give you hair color, hair style, likely race, and the shape of the top of the protesters' ears. While it's not definitive, given that a fuller redaction is easy and has no disadvantages, I don't see why someone shouldn't try.
