This looks like a bug to me. My team has no blue character. When I attack with blue gems it does no damage as the character is missing (this is fine and makes sense). So why is it increasing the mana generation on that enemy I hit with blue gems?
Every tile cleared that hit an enemy will charge their mana, even if you don’t have a hero of that color (those tiles still do 1 HP of damage). It’s simply one of the trade-offs you make when you use multiple heroes of the same color. I think even tiles that miss due to accuracy reduction still charge enemy mana, but I could be wrong on that.