Is There Something Called Blue Jagger In Diamonds? If Yes How Much Would That Cost?

Jager is an old term describing the grade of color of a diamond. Now international diamond trade use letters from D (colorless to Z (fancy yellow). Jager referred to the best color possible, in fact colorless: D.
Blue jager may mean a D color stone with a very strong blue fluorescence which will bring the overall size/color/clarity price down.
This strong fluorescence will make the stone appear cloudy blue under strong sunlight.