For duraluminum, I just use regular grey primer. Mr surfacer 1200, as previously mentioned, or Tamiya primer works well. It's not shiny, like airframe aluminum, so no need to go with black (though nothing says you can't).
For that weathered look on a P-51, etc.., I have read various suggestions. Some guys will go black primer, then airframe aluminum, misted with duraluminum. Or, grey primer with duraluminum misted with a regular (non alclad) silver, such as floquil old silver or MM. I've tried this on a styrene sheet and it gives a good weathered tone.