That will be a waste of time, black never looks better than the day it left the showroom unfortunately.
Yes I was cleaning my matt black bike the other day and, other than it being sans dirt, it looks exactly the same as it did before, no look of it being clean, no shine, just a black bike.
To offer some balance my white genesis flyer which I use for some commuting and locking up when out and about in London, has never been washed since I bought in new in 2008, so us dirty bar stewards come from all backgrounds, black and white

. Actually mine still looks surprisingly white, must be all the British rain. Admittedly the bottom bracket area is a bit caked and its undoubtedly heavier, but I'm hoping its helping achieve an almost un-nickable status. Its grimy-ness has become a badge of honour now and I'm hoping to go a decade with no washing (not me, the bike!) and then maybe I'll see what lies beneath, could be a great frame protector all that c**p
My old Raleigh Max Ogre was like that - I rarely cleaned it, so when people said things like
'Wow, that's a well used bike!'
I took it as a badge of honour. Unfortunately, it eventually rusted up and seized, so I never made that mistake twice!
Because I don't have a black bike! (yet)
You want one?? I can sell you one, it might be a bit big, but hey, you'll 'grow into it!'
All my bikes are currently cleaned, serviced, and immaculate, even the black ones. I look forward to being allowed to get them dirty.
I am currently cleaning them all, including my Mum's.
my black bike is matt black carbon, and for some reason only needs a quick hose to clean the crap off
Yes and it looks exactly the same as it did before, doesn't it?
I washed mine down with Turtle Wax 'Wash And Wax' car stuff as a bit of an experiment (I am using it for all the other bikes too), complete with an assuringly large amount of foam...... And it just looks exactly the same as it did before (except it is clean), I might as well not have bothered!
Oh and the carbon fibre didn't melt in the soap and water either, just to inform you all.
Well, at least I've shamed a few of you into washing the things.
I was going to wash it anyway, but yes, you might have sped the process up a little.....