Board Thread:Roleplaying/@comment-97.118.5.20-20150427033829/@comment-6881629-20150427083957

You don't really join anyone but one side will ignore you depending on if you kill Paarthurnax or not. The Greybeards give you a repeatable mission Find the Word of Power to find word walls (just like the letter from a friend) and Paarthurnax has Meditations on the Words of Power so you can change the boost to one shouts power. the Blades have Dragon Hunting which will send you and the Blades off to kill a random Dragon they also have the one off quests Rebuilding the Blades to find three new Blades and Dragon Research which will give a permanent defense increase against Dragons.

It's too late now i'm afraid but if you stop doing the main quest after finding Sky Haven you can do as many Blades missions as you want before having to choose (have a look at this thread if you want to know more about that).