Some colors are more freindly than others, but I wouldn't give up on black just yet. Generally, 50/50 is thin enough to minimize orange peel, although I've heard of some people having to resort to 60% spirits.

Several "rollerheads" have reported far greater self-leveling properties using a dash of penetrol. I'd certainly try that before considering a color change.

Another factor may be humidity. Has the weather significantly changed since you rolled the blue paint?

Other than that, I'll recommend the obvious just to be sure:
1)Load up the roller as lightly as possible with paint

2)spread the paint as far and as thin as possible while painting a panel... Pretending that Rustoleum costs $1000 per quart and you've got to cover three cars with one can or else your wife gets the waterboard.

Hopefully this helps