IMHO as flock is just a web browser extension and posting a “blogged with flock” button is not likely to be an issue. In fact posting a “blogged with Windows Live Writer or Ecto or Qumana or BlogDesk” button would be the same difference. The point being is that you are not going to be making any money from doing so, right?
When wordpress.com first opened they had a deal with Flock which allowed Flock users to use the site before it was opened up to the general public. So, they’re pretty cosy and I wouldn’t imagine there’d be a problem.