I'm confused; do you want the underquilt for your WB Blackbird or do you want to buy a RidgeRunner as well?
If you want a full-length underquilt for the Blackbird, the Lynx is not for you. I had the same question and was told that it is too slim to fit on a gathered end hammock. So unless you want to get it in combination with the RidgeRunner, forget about it.
As far as full-length underquilts go, I only ever tried the HG Incubator. It works alright with the Blackbird. I had some sag issues, but HG uses a different suspension now. So this might have been fixed. I assume all full-length underquilts will be prone to sagging, simply due to the weight.
To throw another option out there: I just ordered a Wilderness Logics underquilt
It's rated between 3-season and winter (10°F) which is what piqued my interest. I'm chronically cold, and I'm hoping this'll fix it.