Hello!
As far as I'm aware, there are no "popular" S1/S2 servers running Blackwood GP/FBM. Your best bet is racing in the demo servers which are set to BL1/FBM. I would recommend:
[AA] Demo FBM
[AA] Blackwood FBM, but I think they're doing short races (3-5 lap races every few minutes).
There are others which I haven't tried.
FYI, if you are having trouble locating servers running the particular track and car, play around with the filters (see attached screenshot):
Click on FBM (it turns blue = search for hosts allowing the FBM), click on XFG/XRG twice (it turns red = search for hosts disallowing XFG/XRG), Click on "Race (to stop cruise servers showing up), set Empty and Private to [no] and public to [yes]. Hit refresh and you'll the list of servers updated according to your preferred filters. Click on "track" to sort the results according to track (not really needed here as there are not much search results at the moment...)
Click on the "?" at the far right of each host to see the session (i.e. race + number of laps or quali.) and who's connected to the host).