Well, first of all you don't need to make a block bullets brush an entity, it's just the texture. Second, I'm not sure it'll have the desire effect. Block bullets is just an invisible brush that doesn't seal your map or block visibility, unlike player clip, it doesn't allow projectiles and bullets to travel though, making it useful in some situations. However, it looks like you want to make a spawn room that enemies can't fire into? If so you'll want to create a func_spawnroom and func_spawnroomvizualizer brush entities. stretch the func_spawnroom over every where you want your team to be able to change class without dying. In the object properties menu select the desired associated team and name it something. Then cover the door way with the func_respawnroomvizualizer and, in the object properties menu, set the associated respawnroom to the one you just created. Enemies and all enemy fire shouldn't be able to pass the func_respawnroomvizualizer.
Hope I helped in some way.
Edit: I'm dumb, respawnroomvisualizers do not actually block enemy fire.