Having done a prosthetics course, Ive got to say that it isboth expensive, and time consuming to produce a latex mask of the kind your talking about.
what defines a negro face ( forgive if this sounds non pc, its not ment too

) is the shape of the nose, and what i would be inclined to do,is use grease based make up, and a prosthetic nose, and chin.
these two pieces can then be reused, and the make up, can me removed, using wipes, hidden in a towell. oh...and add a wig. remember disguises in the real world, only use small elements.
this is going to be much cheaper.
( your not doing finnians rainbow are you? the change can also be done with tightly focused lighting, and a wig)
you're like Yoda.you dont say much, but what you do say is worth listening to....
Greg Wilson about.... Me.