You can't dye things lighter. Period. End of story.
If your fabric paint is opaque, then you should have no problems with painting over black. If it's not, go buy an opaque version (Neopaque is one brand that comes to mind) and use that.
You could also just strip off the wrapping and redo it. That might be easier -- there are tsukamaki tutorials online that are fairly easy to follow.
Therese von Ludowing (Sound Horizon's Märchen)Elsewhere: Journal ~ ACP ~ CosplayLab
Catelyn Stark (A Song of Ice and Fire / Game of Thrones)
Elisabeth - red ballgown (Elisabeth - Takarazuka version)
Thessaly (Sandman - The Kindly Ones)