The official Blythe website confirms that a new doll in collaboration with CLAMP will be released in the fall:

The latter half the year, in the fall, we are planning a collaboration doll with super popular manga group, “CLAMP”!

My guess is the Yuuko doll that was announced at the CLAMP@MOBiLE Fes! 2011 event.

It looks like this doll will be available for purchase? Unlike the Sakura and Watanuki ones which were only made for the charity event.