The short answer is that JP doesn't have enough skill players this year. A few baseball players who could have helped the team this year opted to not play football. But I doubt the team would be in that much of a different spot at this point of the season had they played.
However, the thoughts that (1) Prep is on the downslide to long-term mediocrity and/or (2) Black is the reason for said downturn are wrong. The current 10th grade class has a few really good players. The 9th grade class is loaded with skill players and linemen. It's the most talented class in the school. JP should be good next year, and really good for the next couple of years after that.
As for Black, it was time for a change. Black's offense had fallen behind the times, his refusal to adapt allowed the program to grow stale, and his well-earned ego and gruff demeanor had eroded morale inside and around the program. Jerrion Ealy's immense talent masked these issues.
Coach Turner isn't the reason JP is having a down year, but like all of us, he's not perfect. He's made mistakes. But he's running a more modern offense and he will soon have the talent in place to fully implement it.
The MAIS of today is more competitive than it has been in years. JA is good this year and was good last year. It was not very good for the 4 or 5 years before that. Hartfield is a new challenge to Prep's ability to lock down Rankin County talent. MRA has the benefits of geography and demographics.
The elephant in the room is the willingness of MAIS schools to admit kids into school who are borderline academically but are top-notch athletically, and the willingness of boosters within those schools to pay for these kids' tuition. Hartfield is heavily recruiting and sponsoring athletes. While the extent to which MRA routinely does so is somewhat exaggerated, MRA certainly recruits and sponsors a few athletes every year. JA is good now - and there are obvious reasons. Pogue is a good coach and he's found ways to get boosters to supplement his roster in the skill positions.
Prep doesn't recruit and sponsor athletes as much as these other institutions. It has always had the raw numbers to allow it to avoid recruiting and sponsorship. Whether it will be willing to fill its talent deficiencies in certain places with sponsored athletes remains to be seen.