Painting galvenzied steel can be done but requires a great deal of surface prep, otherwise it just peels off. Do a google search and you'll get plenty of good info if you choose to go that route. Don't take shortcuts, if you do the paint will peel and you'd have been better off just using them as you bought them
Save yourself the time & headache and find some black brackets. You'll pay a lot more, but you'll be much happier in the long run
Those who can, do...
Those who can't criticize on the internet