Negligible? Really? Have you done this and checked for the difference?
Read the data sheets for ceiling tiles, some time. It makes an audible difference. The wavelength and typical intensity are used to determine the hole size and pattern, for absorption.
From the Armstrong link- "Will painting acoustical ceilings affect acoustic efficiency?
Yes. Armstrong acoustical ceilings can be painted using no bridging or porous paint that does not cover the holes, but expect a slight loss of sound absorption."
http://www.armstrong.com/resclgam/na/ceilings/en/us/article17697.html
As I posted before, why not just buy black tiles?
http://www.ceilume.com/gallery/results.cfm?catid_2=27&gclid=CPCZ2py9l58CFQUeDQodcHi8Aw