Perform the following to get a Katana and easily join the Brotherhood Of Evil. When you get out of the sewers after the emperor's assassination, go west to reach his heir. When you get there, speak with him and give him the necklace, then talk to him again and select "Assistance". He will then unlock his chest so you can take what you want. Make sure you get the new iron armor instead of the old things, the steel claymore, steel bow, and some potions. Also, make sure you are healed and have your Magika full. Then, start to beat up the monk climbing up and down the stairs with your Claymore. Run outside quickly after that. Take out your bow and wait until he comes out, then repeatedly shoot and slash him while healing and he will fall to his death. A message will appear stating than an unknown force has spotted your kill (or something similar to that effect). It is not the guards or a secret force that will kill you. Pick up the monk's weapon, which happens to be a strong Katana, and go back into the building and go to sleep. You should awake to a man in a black hood. He is part of the Brotherhood. Talk to him and he will eventually give you an initiation mission to become part of the Brotherhood. He will also give you a nice dagger.
Instead of killing Jauffre, as he is a main character, just fight with him then collect Martin. When you bring him to the Cloud Ruler temple, accept Jauffre's offer to join the Blades. You will now have access to their armory located in the east wing, directly to the left of the door. Inside you will find a supply of Blades Cuirass, Greaves, Gauntlets, Boots, and Helmets; as well as plenty of steel bows, steel arrows, the Akivari Katana, and the Akivari Dai Katana (two handed).