Some of my first projects really suffered from large streaks and blotches, and subsequently got darker and darker as I tried to cover my mistakes. Then I asked for some help... (before I found this site)
Here is a dyeing technique I have been experimenting with, as told to me by my Father in Law who worked in the leather biz for many years.
He uses a 3 bowl method. 1st bowl has dye in it. The second 2 have water in them.
Wet your leather with a damp sponge. Dip the sponge in some of the dye, then one of the water bowls and apply light coats. The water will allow you to build up layers of color without getting as much of the blotches and stripes. (or they'll be lighter anyway)
I have all but given up using the wool daubers, except for applying gum tragacanth to an edge. I usually buy my sponges from Trader Joe in the kitchen aisle and just cut about a 1" square for dying, then toss it out when I'm done.
This dye method has really worked for me, and I didn't even have to dig ot my old airbrush and get it going... Now if I can figure out that fangled digital camera we have I'll get around to posting some pics.