For architecture models we don't paint them at all but use a tripod and very bright light while adjusting the camera settings accordingly to get great results

Why make them black ?  To resemble the real fossiles (what I assume they are) ?  Shiny black objects have an advantage If you want to catch hard reflections but otherwise I only see disadvantage because it is getting harder to catch the details of the surface. "How to photograph white objects" gets you to a lot of helpful information. Take for instance this link.

http://www.digital-photo-secrets.com/tip/4593/white-white-images/

Apologies for the untimely response. I have whitened silicone, plaster and even resin casts (recently made as well as from old museum collections) using ammonium chloride (NH4Cl). I never painted any of them (the silicone casts are light grey) and they all photographed just fine. See the attached figure as an example. Bear in mind that each of the trilobite specimens in these photos are no more than 10 mm long.

Thanks Allart. Your photographs are beautiful! I too have some tiny trilobites to photograph, working on proetids at the moment. What macro-photography apparatus do you use? We have good old fashioned Leitz Aristophot in Leicester with the original Summar lens; tried and tested over many years. I have also had good success with whitnening dark silicone and resin using NH4Cl. The problem is that I have made brilliant white dental plaster casts (not so brilliant for photographing) from silicone moulds I made of museum specimens. I was using an old tub of photographic opaque to blacken them, but it ran out and is no longer manufactured. Any advice would be greatly appreciated.

Ah I see. You might want to try a thin pointed permanent marker (e.g., Edding) if you can reach all the areas of the trilobites easily. Alternately you could try fountain pen ink (old fashioned inkpot) but I don't know what the long term effects will be.

As for the photo apparatus, I am using an ordinary Canon EOS 400D. The lens is more important: a second hand Leitz macro objective that I was able to source an adapter ring for which allows it to be used on my Canon.

This looks to me like a classic problem of photographing a white surface with B&W film. 

They key (even in today's electronic world) is to light your subject with low angle light, which will enhance shadows in a textured surface.  By balancing the strength of the light source with the lesser intensity reflected light, one can obtain a wide range of shades of grey in an otherwise all-white object. 

The overall total balance can then be adjusted by simply increasing or decreasing the total light coming into the camera by adjusting either f-stop or shutter speed. 

Since you seem to want maximum detail in the picture, you should use the small end of your f-stop spectrum available to you to achieve maximum depth-of-field . 

Update: I have had some recent success with using India ink to colour the plaster dark grey, which makes it suitable for photographing. I wish I had known this sooner; would've saved a lot of painting time! Thanks for the advice F. Louis Floyd. I use the smallest aperture possible to achieve maximum depth of field and low angle lighting, but as the specimens are whitened with ammonium chloride sublimate, it is still necessary for them to be blackened first to achieve maximum contrast.

Materials and raw materials are very many, some of them homemade .. Are you looking for something specific?