This looks like a hematoma - and it doesn't look too swollen, which is good. This blood will likely just be reabsorbed. IF it is very swollen - OR if it becomes more swollen, then you want to go to the vet right away.
It's like - when you get a very bad bruise (or, have a post-surgical bruise), a bruise is actually a little bit of bleeding of the tiny vessels - they stop, and the bruise goes away in time.
Keep an eye on it and just make sure it doesn't continue to swell.
