Yes, of course

As Stompin' Stompa said, black looks really good under silver.
This being said, you may also want to consider two other options:
1. If you want to paint your Grey Knights non-metallic, white primer (any brand) or Uniform Grey primer from Army Painter might be easier to work with, depending on how light a grey you're looking for. It's much easier to get white primer to go almost black, than black primer to go almost white.
2. If you want to paint your Grey Knights metallic silver, you may wish to consider Army Painter's Plate Mail Spray Primer. It's a metallic (silver) primer, and if you're after a quick, nice looking army, it's a huge timesaver.
In my opinion,
GW Leadbelcher is the route to go, to cover black primer. It's nice and thin, and has excellent adhesion and coverage.