Here's one method (not certain it's the best, but it works pretty good) :
1. Spray the gun matte black.
2. Drybrush some very dark metallic, like Gunmetal or Tin Bitz from (use Games Workshop "Citadel Color", Tamiya paints or whatever you want)
3. Either wash or airbrush a clear-blue color (again, use Tamiya or a very thinned out ink from (for example) Citadel Color). The Citadel ink gives you pretty good light refraction when viewed in different angles.
/Joe F