Primer for enamels?

Acrylic will do just fine. I use cans because my favourite primer is an automotive one and is only available in cans. Not that I'm complaining.. results always have been excellent.

Primers for choice is really what ever you find. Testors has a primer for their paints and Floquil (made by testors) also has a primer in which you can paint enamels over. I have been using Testors primer for my acrylic paints. So far so good. I also have used Kylon primer. Most everyone will tell you that Testors and Floquil are good modeling paint primers. You can use a lacquer then use enamel paints also. If I am using Testors MM or Tamiya Acrylics, I use Testors primer. I am not an expert but those are some of the things I was told. If I am wrong on this, I hope someone corrects me because this is what I been doing. I have not primed too many things especially when I use enamels.

How about priming with black on an aircraft? Would that accent the panel lines? Or is it just better to apply a wash for that?

A lot of people pre-shade in recesses, but that's usually done with an airbrush over a lighter primer coat... The lighter primer coat (white or grey) is often used 'cause il allows you to spot imperfections in the joints, filler, putty, re-engraving,... But nothing's stopping you from trying it out. I often do it on AFVs...