Just got a response from Daniel Stern about the blue tint on the glass of the Philips X-TremeVision 9006s. Looks like I was correct in my assumption that it's not a detriment to performance in this case. He did a much better job of explaining it than I did, so I'll just post his response below.
"Any of the blue-glass "extra white" bulbs is a nonstarter as
far as seeing better is concerned. Sylvania
Silverstar/Ultra, PIAA, Hoen,
BlueVision, CrystalVision, TruView, Nokya, Polarg, etc. --
all the same scam. They produce _less_ light than ordinary
bulbs and have a very short life due to the light blocked by
the blue glass. All it does is change the beam color, which
doesn't help anything (except arbitrary appearance
fashion/style).
Now, what about the blue ring just below the tip of ultra
high output +80/90/100 bulbs like the Philips Xtreme Power,
GE Night Hawk Platinum, Tungsram Megalicht +100, etc? That's
actually completely different. The marketers say this is
there to make a fashion statement with your headlamps, and
yes, from various off-axis angles as you observe the
headlamp, you might see some blue glint, but it doesn't tint
the light in the actual beam, and that's not the actual
reason it's there. It's really there to cut the mean
spherical output of the bulb down to legal levels as tested
in an integrating sphere (the machine used to measure light
output from a bulb to determine if it falls within the
legally required range). The blue ring filters a part of the
bulb that has nothing to do with beam formation because it's
not located between the filament and the reflector. Gives
the kids a blue flash they can point to from certain angles
without colouring the beam or filtering out any usable
light. Meanwhile, the filament is pumping mad lumens through
the uncoloured glass where the reflector is looking. Clever
trick. It works."