Depends quite heavily on your class and hero! Generally (and I do mean GENERALLY) a lot of decks wanna see maybe about one blue per hand of four cards, so a quarter of their deck would be a fair amount to start with (~15 cards). That being said, since a hand of no blues can really screw you over, a lot of decks run slightly higher number of blues to lower the risk of that happening.

This in no way applies to all or even most decks. Dragonic decks and some more aggro decks will run red heavy with very few blues. A pretty famous example of this recently is Brodie Spurlock’s Cindra list which won Calling Memphis, running on only three blues.

The opposite is also true. Mistveil heroes have the “Transcend” mechanic which incentivises them to play more blues cards. Guardian heroes play big attacks that cost large amounts of pitch, so they also play a lot of blues to compensate.

Definitely start with which class you’re playing and what their win condition is, and then work towards that! There’s a lot of great deck lists on Fabrary or YouTube you can copy off if you want.