Chapter 28 of Order of the Phoenix makes it extremely clear that Umbridge believes that Sirius Black is working with Harry Potter.
“Mr. Potter,” said Umbridge, “let me remind you that it was I who almost caught the criminal Black in the Gryffindor fire in October. I know perfectly well it was you he was meeting and if I had had any proof neither of you would be at large today, I promise you. I repeat, Mr. Potter ... Where is Sirius Black?”
However, this is a clear contradiction of the official story, that Sirius Black is a highly ranked Death Eater. For example, the headline of the Daily Prophet article on the Azkaban breakout was "MASS BREAKOUT FROM AZKABAN MINISTRY FEARS BLACK IS "RALLYING POINT" FOR OLD DEATH EATERS" (OotP, chapter 25) and this article had several quotes from Fudge supporting this view.
So, do we know what Umbridge actually believed? I can't see an easy way to believe both of these at once.