As a follow-up to my green pen comparison, here's a comparison of the blue pens currently in my collection:
I'm not usually a fan of blue ink, but because blue pens are so ubiquitous, it's been impossible to avoid acquiring quite a few of them. The Bic ballpoint, which must be the most ubiquitous of them all, writes with what I consider to be the default blue ink, somewhat similar to the blue colour of denim: it goes with everything, but isn't particularly exciting or helpful if you want to make a more unique style statement.
Because of that, I tend to prefer the blues that are the most different from the default. The sky blue Pentel Slicci is probably my favourite on this list: it's a bright turquoise-y blue that is also dark enough to read easily, even with that fine of a line width. Another favourite is the blue of the Sharpie Pen (the RT version in this post); on its own, it looks fairly standard, but when compared to other blues it looks slightly greenish. The Centropen Liner's blue is similar to that of the Sharpie and writes with a finer line, but it is certainly not as widely available in North America. The lavender purple Dong-A Miffy is not quite blue, but then it's not quite purple either, and it is also one of my favourites, partly because it is so different and helps to fill that elusive gap between blue and purple. My other favourite is the greyish blue of the Pilot FriXion Point, even though I've now realized that it's actually quite similar to the Bic ballpoint blue.
The only blue on this list that I really don't like is the aqua Staedtler Triplus Fineliner; it's simply too light for me. I don't usually want a deep dark blue colour, but the Pilot Hi-tecpoint V5 or the royal blue Fineliner would be good choices for that, or the blue Pentel Slicci if you want something with a finer tip.
Do you like to write in blue? What are your favourite blue pens and inks?