Akismet is very good at detecting comment spam. If it were any good at detecting signup spam then wordpress.com would not have so many spam blogs.
I also would track down spam blogs there. Sometimes manually through search engines and a curated list of known terms and sometimes with tools that one of the devs created for me.
I suspended thousands of genuine spam blogs. Sometims mistakes were made but they were rare.
Then, although some automated tools were created to try and stem the tide, that hunting and suspending was deemed not to be a priority. It was important at the start that wordpress.com was seen to be clean so it could grow but once deemed big enough, it was stopped. While I know I was using a supersoaker to put out a burning car it didn't take long and I found it satisfying
This time of year I'd be hunting hunting Halloween spam blogs and would start to see Christmas spam blogs too.