Left Bundle branch block is more critical as the block comes from inside your left ventricle. However, it is not disqualifying by itself. The FAA will probably ask for more tests to be done, i.e.: stress test, holter monitor.
In a nutshell, there is an electrical block that makes a heartbeat not go all the way through your heart. I would seek the consultaion of a good EP (electrophysiologist) they will be able to asses your situation and if need be write a letter to the faa.
My parents were both doctors and I learned a lot about cardiology growing up, I don't think this alone should hold you back from getting your first class.