To elaborate, some 757-200 aircraft that are configured with the international BF cabin are also flown on domestic routes, such as EWR-FLL/PBI. You get regular domestic F service, but a BF seat, and sometimes they'll run the IFE loop.
The only way to tell which flights offer this configuration online is to look at the seatmaps -- 4 rows of 4 seats indicates a BF aircraft. 6 rows indicates a regular domestic cabin, although I believe CO is slowly reconfiguring all 757-200s with BF cabins. Not sure of the status. I believe that on the printed CO timetables (i.e., the PDF), the BF planes are listed as "752" and the domestic planes are "757"

Now, the wrinkle is that co.com has been listing the premium cabin on its 757-300 aircraft as "BusinessFirst". This is a website bug. ALL 757-300 aircraft are configured with domestic FC seats, either 3 or 6 rows (with all going to 6 rows -- again, not sure of status).