I've experienced the blue glow (floating on the glass) on numerous 300B tubes to varying degrees and never found a correlation between such VS sound quality or longevity.
The only high failure rate 300B's I used were from Svetlana St. Petersburg, though I did have one pair make it beyond 600 hours of usage while still testing as new.
The blue glow, to varying degrees, was evident in 300B's from China, Russia, Eastern Europe and 2 pairs of WE's manufactured well before the late 90's.
The color I did see prior to failure was increased amber/orange, but it was internal from the grid wires and it did not coat (float on) the glass envelope like the blue did.
What was interesting about the "failing" Svetlana's was that if they did not completely fail they still worked (sounded fine) at much lower SLP's (even though maybe half of the grid wires were glowing/operational).
Testing them on a tube tester @ that point (TV7/539C) would put them out of their misery.
This said, I agree with ghd's advice as they are new tubes form a vendor.
DeKay