Short answer:
The current technologies for blue (or white) LEDs have about 4.0V forward voltage. Too much, I'm afraid, to run directly off of a pair of D-cell (i.e. ~3VDC).

Long answer:
We, too, like blue LEDs. We toyed with the idea of adding blue LEDs to Tree1.1. We could have added a power supply circuit that converted the battery voltage rail to a higher voltage to run blue LEDs. However, that would have added cost
and complexity to the design. It also would have reduced the battery life. In the end, we decided that a simple, cheap design that ran for a long time off of a pair of D-cells was more important.

We keep watching the blue LEDs and when they get down to the same forward voltage that the traditional red, green, orange & yellow LEDs have (about 2V), rest assured, we'll put them in a PicMas tree. :-)