scratch resistant matt black paint spray for chrome taps
The two approaches that might actually work (both rather expensive) are to replace the taps and fittings with factory black ones, or to send the ones you have now out for replating with a black nickel process.

Paint on chrome is a failure that usually does not wait to happen.

I guess there is the option of "Plasti-Dip" - a rubbery spray coating that is commonly used by auto enthusiasts to recolor and/or protect chrome parts - in a somewhat reversible manner (it can be peeled off.) Being rubbery it does not chip, and reports are that it holds up pretty well until deliberately peeled off. Here's a link (no association and I'm not really a fan of the stuff, despite being aware of it.) 

http://www.eastwood.com/blog/tech-articles/plasti-dip-faqs-frequently-asked-questions-about-dipping-cars/