I am wondering if you had too much smoke. The ribs were not tender, so they needed more time. But you said they were very dark, so how did they get that way? Smoke is the only thing I can think of, unless your temps were so high that it burnt the meat. That makes me think your temp gauge might be fine, or at least close. Did you run on propane only, or were you adding chunks/chips? If yes to chips/chunks, how much and how often? Did you wrap in foil toward end of cook, or unwrapped the entire cook?
At 275, spares an average size spare will cook in 4 to 4.25 hours. I seldom do baby backs, but as I remember, they cooked slightly faster (maybe 20 minutes less).
If you care to try again, I would do around two hours or so ( until they are a nice mohogany color) with light smoke. Then wrap in foil with the typical honey/brown sugar and whatever else you want and go for another two hours until the ribs are almost done. Then glaze both sides and go for 5 minutes to set the sauce.
Eat.
S.