Xenofarm does no intentional blacklisting, but there is a bug or eight when it comes to handling broken result packages and similar, that has the effect of discriminating some machines, just the same. If possible and given the incitament and time we try to work them out and improve on the system, and welcome the help from motivated people too. :-)
Good places to peek are /pike/data/pikefarm/ (at pike.ida.liu.se), and reviewing the logs in /pike/home/xenofarm/
What is the status of this? Is there still other people whose pikefarm results don't appear on the website?
I sent some (7.4, 7.6, 7.7) today with a darwin 7.4.0 machine (E-T.linkeo.intra) which don't shows up in the list, if this can help to track down the bug.