I wouldn't necessarily describe it as a Chevrolet "screw up", as the various program teams can assign the VIN structure as they see fit. In other words, they could choose to use a separate VIN structure for various engines, or assign a specific VIN for "SS" models (or not), usually for warranty tracking purposes.
"Chevrolet" may have decided that it was not significant to track Blazers separately from pickups and may have authorized this particular VIN structure as a conscious decision, or so they could appropriate that space for some other purpose.
...OR...
Maybe St Louis screwed up and assigned the pickup codes to the Blazer body types, in ignorance or defiance of direction from Central Office. It would be interesting to know if the other VINs you saw that were like this were all from St Louis only, or if the other assembly plants coded them this way as well.
K