I've been slowly building an 40k Undivided Chaos Daemon army to use with my Nightlords in Apocalypse games.
To create Undivided Bloodletters, I cut off the back of the Bloodletter's head's and sculpted hair to look similar to the Furie models.
The Bloodletters are being painted the same way as the Furies.
The explanation for this army is that some Furies have grown stronger by eating enough souls or eating actual Khorne Bloodletters to gained their appearance.
This is a leftover Bloodletter from a Bloodcrusher set. To convert the model, I cut off the hand that attaches to the Bloodcrusher's collar and replaced it with a Crypt ghoul's hand, sculpted the hair and pinned it to a skull glued to the base.