You don't get many just blue rugby boots, but you do see the colour used a lot, especially on the Predator Rugby X boots. The blue is used to compliment black or silver boots, and can often be found on the sole of the boots too. Blue is a great accent colour for your boots.
Coming from light at a wavelength of roughly 440-490 nm, blue is one of the additive primary colours, the term covers a range of tones from navy blue to cyan, pigments include azurite, ultramarine, cerulean and cobalt.
It could be said that blue was one of the more acceptable colours on rugby boots for traditionalists, partly due to it's used mainly as an accent, also because darker shades stand out less, if you want a blue pair of boots nobody is going to think twice if you are wearing them.