While cool in theory, this seems like a horrible idea, especially given that this is a rythm game that requires tight timing.
Given that motion blur has to be done trough post processing there is no way to have it without adding at the very least a slight delay, and gameplay wise it won't make much of any sense either, since you want to see what you're supposed to see ON time.
Not with delay artifacting(, which is essentially what motion blur is).
I once had an idea to add a game-wide bloom/glow effect, but that falls under the same category, though it's not a delayed artifact.
These are generally bad ideas for a game like osu!.
Nothing personal, just trying to explain why this wouldn't be a good idea.