The blue shirt rule is not really how they are doing it:
13.9.1 Letter of Intent Limitation. Each SEC member institution is limited to signing 25 football prospective student-athletes to a National Letter of Intent, Conference financial aid agreement and/or institutional offer of athletics financial aid from December 1 through May 31st of each year. [Adopted 5/29/09; effective immediately; revised 6/3/11; effective August 1, 2011]
(Heres a .pdf link if you want to peruse the SEC rules yourself.)
The dates are critical, because the SEC bylaw collides with a new NCAA rules interpretation that impacted this recruiting cycle. Academically eligible student-athletes who plan to enroll early (in January) are now allowed to sign aid agreements with universities as early as Aug. 1.
Tennessee had a flood of players sign aid agreements last month perhaps a half-dozen players or more. Why is this significant? By the letter of the law, those players wouldnt count against the SECs signing limit. Theyre freebies, if you will.
Its the new NCAA Aug 1 rule. This allows schools to skirt the SEC rule. But they still can't go over the 85 so they'll either have to gray shirt some or let go of upper classmen.