Blue speedos are probably the 2nd most common speedo colour after black I would think.  That rings pretty true when it comes to my speedo collection over the years, mostly black speedos, then blue then the next most significant is a long way down in the count.

The speedos that I most often wear to the pool when swimming laps are my blue AussieBum classic speedos. They are nylon, a snug fit but not too tight and I think they look pretty good and reasonably discrete.

These are those blue AussieBums.

And here are some guys doing ‘naughty’ things in their blue speedos.