Blues South struck back just before the half hour mark through a try by captain Shane Young with Williams converting. However, the comeback seemed well and truly off when Bartle was red carded for stamping two minutes later, before Joseph Thomas crossed for the Eyasses third try of the first half, Price successful with the boot again to send his team in 19-10 up at the break.
Despite being a man short, Blues South showed good spirit in the second half, Williams' second penalty early on keeping them in touch, before Ellis Wyn Benham dotted down, Williams' conversion putting them ahead for the first time on the night on 54 minutes.
The Eyasses tried to overturn the lead with time running down, but a yellow card in the closing stage to Jonathen Fox hampered their efforts as the hosts held on for the win.
Ospreys U16's must now pick themselves up for Sunday's game against RGC1404 at Corus RFC (1.30pm), while the U18's entertain the North Wales team at Llandarcy Academy of Sport (1.00pm).