Magisters do indeed practice blood magic. It's like weed, it's illegal but everybody does it.
They could achieve so much more all the mages if they cooperated more nad fought among each other less.
Possibly, but why would they? A lot of humans don't care about cooperating for greater achievements, but on mantaining/increasing their personal powers and privileges. The magisters don't want to lose their position or their privileges.
Many believe that the Tevinter magocracy means that all mages enjoy the same rights, freedom and possibilities. That's not true. While they're treated better than in the White Chantry countries, most are still under control of someone. Just mages and not mundanes.
Yeah, that "ban" on blood magic strikes me as pure lip-service. I guess even the Black Divine has their standards, albeit not many.
Asunder can clear somehow this for you (I remember you haven't read yet?).