Just did BBNP as well as BB Ranch State Park. Was in the area from Feb 20 - 27. All the developed campgrounds were full and many of the primitive road campsites were full. Spring break is starting and the rangers said they would get even busier thru the month of March.
As was stated earlier in the thread, the primitive sites can only be reserved once you're in the park at Panther Junction. I was worried I'd be turned away as I listened to the 3 people ahead of me get sent to the northern most sites. These folks walked up and just stated they wanted a site with nothing in particular specified. It seems I lucked out as I had printed out their Backcountry Planning Worksheet (
https://www.nps.gov/bibe/planyourvisit/upload/BC_worksheet.png) and had my first two choices for 6 days written down. This seemed to make the ranger try a little harder for me and with a little flexibility, I got 5 of the 6 sites I wanted. I actually came back after my third night and was able to swap a site on day 5. So I guess what I'm saying is, do the worksheet and be ready when you get there.
BB Ranch State Park is even more remote and very scenic as well. It's 27 miles of dirt just to get into the park HQ to get a campsite reservation. From there I'd estimate there are another 50-75 miles of trails with multiple campsites. They are all primitive with a shelter and fire ring only so you have to be self supporting. There are hot showers, restrooms and potable water at the HQ. Bring your own firewood as non is available at the park.
Plan and enjoy.......