For the best contrast you need a steel with an appreciable amount of nickel. Steel like 15n20, L6 and 203e are best for the "bright" part of a pattern weld. For the deep "black" 1095, 1084, 1075, 01, W1 and W2 are great. Mild steel etches out a gray color.
Keep in mind that the contrast will be greatest on a hardened material. I don't know why, but martensitic steel etches blacker than than unhardened stuff.
I use 15n20/1084 in pretty much everything. Balances out to .79% carbon, easy to heat treat and very easy to move around with the hammer.
If you can have a pro make a billet with you or take a class, you can shave a significant amount of time off the learning curve.