This question has irked me for years, so I figured I would ask it here in case anyone has an in-lore explanation for it. It doesn't make sense to me that an organisation of assassins would wear uniforms when outside of their sanctuaries, even more so because we know that if a single assassin is caught or compromised it could mean the death of the whole sanctuary (whether because the assassin reveals its location, or because the Purification ritual needs to be performed to safeguard the Brotherhood's secrets and other branches).
So why bother with clearly-identifiable outfits out in the world? Fear? It seems like the fear associated with being able to identify which organisation a fleeing assassin belongs to would be far outweighed by the fact that you can clearly identify that fleeing assassin in the first place. Surely if the Dark Brotherhood wants their assassinations to be identifiable to them they could just leave a calling card of sorts next to the body.
Outfits make sense within their sanctuaries, such as robes or whatever else, but outside it seems to make most sense to use whatever is necessary to blend in. If you're infiltrating a castle, perhaps a suit of armour to blend in with the guards, or the clothes of a handmaiden to get close to the target. What doesn't make sense is to be gallivanting around with a sinister-looking black and red leather outfit that covers your face and has knives and daggers on it.
Obviously assassin outfits look cool, I know, so the out-of-lore explanation for this is clear (although they still could've at least vaguely made the outfit blend in with the environment, like the classic Assassin's Creed outfits vaguely do if you squint your eyes). But I'm wondering if we have an in-lore explanation for why the Brotherhood go out of their way to make their lives harder.