First up, a Large Blue Butterfly on this British stamp from 2018 that is from a set titled Reintroduced Species.
Next is a (mostly) blue international stamp from the Netherlands from a series of Dutch Icons.
Here is a definitive stamp from the 3rd series of the Russian Federation. This stamp is representative of industry.
And finally, from Taiwan, one from a set on blue and white pottery.
Click here for more links to blue postage.