The blue comes from the prime pigment of Panzer Grey which is Lampblack.
I'd like to elaborate...
Correct color has been a constant learning process for me. I have looked at the US Army R&D tech reports on their own colors as well as Brit intelligence reports on the German paint industry, and it's difficult to come to a conclusion as to what is right, and what is wrong. Some conclusions are:
1) All the countries (minus US) had the same technology for color control...with the US having a slight lead. However that lead was invested in uniform dyes...so wrt paint, all is the same. And the technology was quite primitive and completely subjective.
2) For US OD & dark matte colors in general, the color control technology was found to be of little value
3) In the US at least, matte colors were for wartime camouflage purposes, and color was not of primary or secondary concern. The biggest concern by a wide margin was corrosion control. The Silver Medal went to IR camouflage formulations & color got the Bronze Medal. This all holds for US, Brit & German industry.
4) Pigments are an interesting point. The Germans were lucky because the premier ochre, sienna & umber mines were all Axis! Spain, Italy, France, Turkey. So they certainly weren't hurting for raw materials...whether they could afford them or had other procurement issues...I don't know. For example, it don't matter if you can get the best French Ochre or Turkish Umber is your linseed oil factories are being bombed.
5) Going back to quality control & the lack of...I was stunned however that when I tested the paint from a WW2 can of OD, & compared it with the color control chip from a completely independent source...the delta E was a phenomenal 0.4! That's a big point for the guys who want to believe in high quality control (in other words, there is only 1 correct color for anything).
6) wrt Panzer Grey, you can imagine if the Americans were telling of a hard time with dark matte colors (like US OD), it must have been far worse for the Germans and their greys. However, see 5) above^&grin
7) If you look at a pre-1950 Munsell Book of Color, it's interesting to see that all the Grey neutrals are really a bluish grey. It goes to show the thinking of 'grey' at the time. Fortunately all the Munsell chips were measured by that expensive technology (which got put to use in American dyes during the war), so there isn't any question that the colors have changed over time. I've even done random samplings out of the 1929 Munsell Book of Color, and the colors have remained remarkably stable.
some ramblings