The "blue hue" is probably caused by the mercury vapour discharge. HPS lamps contain sodium and mercury, the mercury helping to balance the colour output and also helping vaporise the sodium. This is why HPS lamps glow a white(r) colour before warming to yellow. I have noticed as HPS lamps age and start to go pink, the "mercury phase" where they glow white lasts longer (up to a minute or more for some of the lamps outside my house which must nearly be duff now) or, in some cases as Phosco152 says they never leave the mercury discharge phase. I personally have seen this only once so far, and the lamp seemed to phase between the whiter mercury discharge and sodium yellow as and when it pleased, and I think it also had a tendency to cycle. It looks like the council may have fixed it, most likely as it was reported - when I was a boy we lived across the road from a cycling lamp and it drove my mum mental, so I imagine they get reported probably quicker than fully blown lamps. If your MRL6 isn't cycling and the council don't do regular daytime switch-on-and-check of the lanterns it'll probably be like that for a while, essentially a gutsy mercury lamp. It may also be possible that even the mercury isn't vaporising and what you're actually seeing is purely an arc through the Xenon fill gas, personally I think this is rather unlikely but if it is true it will probably knacker the igniter.
I spectro'd a HPS lamp recently and when you break it down you can really see how many emission lines there are.
The orange is the sodium D line and the green is also sodium, this line is weak to non existent in low pressure but as you up the pressure this line becomes more prominent. Anything left of that, so all the blue and violet lines, are from mercury. You can do this with any CD that has had the top label coating removed.
As much as I've said in the past that I'm not too fond of HPS, I am beginning to change my mind. Ever since getting one of my own to play with I've found that a good quality lamp with not too many hours on it gives nice golden light with - surprisingly - very good colour rendering. It becomes apparent though when lamps age as they go pink, much like mercury lamps go green, and it seems that a lot on the street which are worn out don't do the technology much justice.
This video taught me a lot about the science behind HPS:
https://www.youtube.com/watch?v=hArNwYgRgL4