Rectifying a bad bleach job
Dread Maintenance
I don't know if this will be helpful, but I've dyed my dreads lighter a few times. I don't use bleach, instead I use Loreal Technique Excellence for dark hair only with 40 developer. You won't get as much lift and it's a bit of a slower process, but its turned my daaark dark brown hair into a nice light brown with lovely blonde highlights. 
Each time it gets lighter and lighter and more blonde. I'm really hoping to go platinum someday. I get it at Sally Beauty or whatever the name is so hopefully there's one in your area. There's also awesome red shades available. Hope I helped! I know how big of a pain dying your own hair is, but very satisfying when you get good results. Best of luck!