well, of course there's several ways to interprete shivan technology.
However, being a shivan (*1), i can assure you that the stuff the ships are mainly built of is metal. I'm not going into details here, but metal tends (even when painted) to be shiny. Whereas the ships of the scouting fleet were only weakly armoured they revealed more of the actual ship's inner workings, a look at the FS1 textures will show what I mean. As the GTVA started to put more resistance than expected, the situation actually needed some attack ships, which, of course, need to be armoured. Look at the shivan fighter maps from FS2. They still have the black parts, although they're mostly covered by heavy metal plates. Of course metallic hull plating makes sense:
1. metals have a very high melting temperatur
2. temperature will be distributed to the whole metal plate in very short time
3. metal can be reinforced if you put several layers onto each other and melt them together
4. metals have a very very high density
5. you can mix metals any way you want to achieve special effects; only metals are capable of 'metal bondings'.
Any metal also has the feature that the electrons can be stimulated to higher energy levels quite easily (because they can be moved around freely between the metal protons, which also provides a good conduction) so they can absorb and reflect a lot of light, they reflect preferrably light of the visual spectrum, which will cause them to shine.
--> FS1's shivans are not really shiny and death-black
--> FS2's shivans are not shiny either, but they have shiny armour parts (which are not even in the original texture black)
*1 : ask the Sheep or Ellanis 
