Never tried that cross but I put two Black Welsh sheep to a Zwartbles ram year before last and the lambs were great - very hardy and tough (Welsh influence) with good growth rates (Zwartbles influence). They killed out well too - leaner than my OH's Berrichon X Welsh lambs that were done at the same time and the meat was more tender.
My OH popped two of his old Welsh ewes in with my Zwartbles ram this year to keep him company until I put my girls in (on the assumption that they wouldn't come into season until after they were removed - they usually start cycling in November) and he's served one of them so I'm quite looking forward to seeing what she produces.
Sooooooo after that long winded bit, I think a Zwartbles on a Kerry Hill should produce a nice hardy lamb with good growth rates and a nice taste. Just be aware it will probably be plain black so won't be too attractive at market if sold live.