Unless BC has changed their sizing sometime between June 2004 (the last time I tried on a pair) and now...
27 stretch Sevens/stretch TR = 28 in stretch/29 in non-stretch Blue Cult
I find Blue Cult to run REALLY small, and generally not be for curvy gals. The sizes may run differently if you're on the slimmer side. The reason I always went up sizes was because otherwise the fabric wouldn't cover my butt.
I agree with Lilykind Blue Cults run small, but I like the fit. I am sort of lacking in the hip category and I find they fit well. I wear 26 in non-stretch sevens and I have a pair of 26 stretch blue cults, but when I tried on non-stretch blue cults I had to size up to a 27. HTH