Blue stands out on forms/documents done up in black ink and is easier to read and harder to fake. Black pen writing can be hard to read on such forms and is easy to fake. Purple is for wankers, green is just ... just .... not done! And don't get me started on red ink.

I love blue pens. Dark blue pens say "look at my lovely writing people". Lighter blue says "I'm barely touching the page. I don't give a fuck. I don't like writing".

blue black ink is very nice, as is green black, sepia, and Emperor or Chinese red