Three questions -

1) What's the best way to blacken aluminum parts?

2) What the best way to treat badly pitted steel before rebluing?

3) Is there a reasonably good DIY Parkerizing finish available?

I had some aluminum gun parts I wanted blackened and since I didn't have easy access to anodizing material, I went with brownell's moly teflon:

http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1145&title=TEFLON/MOLY%20OVEN%20CURE,%20GUN%20FINISH

I first tried it years ago and since then have used it on many gun frames, slides, parts, etc (steel, ss, aluminum, etc). When applied correctly that stuff is tough as nails and looks really nice. I've even used it on knives. I sprayed it on conservatively with a blow dryer nearby to help with the air drying process. Once air dried (a minute or two), it can be handled by hand so it made it easier for me to coat since I didn't have to hang it while spraying. I sprayed and blow dried a little and continued adding coats until I was satisfied, then baked it in the oven. Others have used gun kote with equal success. I believe you can buy both as a liquid to use with an airbrush if you have one which will result in a better and cleaner finish, but the aerosol can has worked good for me.