Well, you're not talking about an expensive instrument or (no offense intended) one that's prized for it's history or special features. If it's the appearance you're concerned with, I wonder if your best bet would be to lacquer it with a decent quality spray can lacquer you could pick up in Home Depot, Lowes, or a hardware store. I don't know what the best surface preparation would be for the nickel, but it might be pretty straightforward. A black tenor horn might look pretty cool -- especially if you masked off some of the areas (joints? ferules? water keys? valve caps?) to retain the nickel plate there.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)