Just a black coloured mixed breed.
To strenghten either breed I would suggest to get pure blood lines, that are not related to the ones you have.
Just randomly mixing with another breed can ruin everything and lessen the already tight gene pool even more, as these alien genes will pop up uncontrollably over many decades to come.
It often seems the quick fix for rare breeds and I have seen people and even breeders do it many times. But although the resulting F1 might appear to be an enhancement at first glance, breeding these will have a wide array of genetic expressions that will not fit with the desired/intended results.
For your fibro EE project the unexpected and obviously mixed hatchlings might come in handy.