"So they also seemed to artificially glaze the blocks. Note it will weather off over time so only protected random spots left."
Do you think there's a link to the Boxes of the Serapeum? There's some evidence of boxes at varying degrees of finish from rough cut, to smoothed, to what has typically been included in the evidence of 'vitrification' but that one box that show's drips at the junction of the side wall and the bottom of a lid where it appears some coating was applied to the side walls and has dripped down and pooled on the underside of the edge, seems similar to what you're describing.
See in vid at 10:30