You could use a two part epoxy paint. Might be thicker than you want. You can also anodize aluminum black.
Here is a good source of information on working with epoxy and aluminum.http://westsystem.com/
If you want to get fancy you could use a very thin-walled section of aluminum or perhaps even softwood and then give it a thin coat of epoxy and wrap it in a very light cloth of fibreglass or spectra or something like that. Even hemp, if your into that. Polyester resin will not bond well to aluminum, but epoxy will bond really well, and adjusts to temperature the same as aluminum also I think, and it will seal wood and absorb in about 1/8". Use some very fine wet-dry sandpaper to scuff up critical joints before you epoxy it. Of course you can buy carbon-fibre tubes now in many sizes. I think it would be neat to make a pack-frame out of bone dry spruce or cedar branches sealed with epoxy, maybe fastened together with whipping twine sealed with epoxy. Wouldn't be all that much heavier than aluminum when done, and it might look way cool espcially if it has natural curves and tapers. Wood and natural fibres are fairly competive strength and weight wise once they are dry and sealed, but I don't think any traditional natural resins are quite as good as epoxy though. Softwood and epoxy is a wicked combination.
Here is some food for thought...http://www.primitiveways.com/pack_frame.html