The engineers wanted to improve the colour on the chip. "It was the same thing with the Apple and Atari computers," explains [Bob] Yannes. "You would get interaction between the luminance signal, which is the black and white information, and the colour signal. So you would end up with these various colours on the screen which weren't really what you wanted, but it was just the nature of the NTSC video standard that the luminance and chrominance signals interact with each other." To purify the colours, [Al] Charpentier made a risky last minute change. "Al had the idea that if the two clocks were independent from each other, then that interaction wouldn't happen," says Yannes. "We separated the clock generators on the VIC chip so that the colour crystal was a separate clock from the video shift rate." The engineers noticed the default white on dark blue screen no longer looked appealing. "Even though it has good contrast, the transition from blue to white produced kind of an ugly edge," explains Yannes. "We ended up having to make it light blue on dark blue."
— Commodore: A Company on the Edge by Brian Bagnall (Page 412)
Bob Yannes (designer of the SID chip) explains why the Commodore 64 had default colours of blue on blue.