Refers to youthful energy, passion, or hot-bloodedness. Often used in set phrases like 血気盛ん or 血気にはやる.
Young people are full of vigor.
I acted recklessly, carried away by youthful ardor.
血気 specifically means hot-bloodedness or reckless vigor, while 血 in this sense more broadly refers to innate temperament or passion.
Compound of 血 (blood) and 気 (spirit/energy), literally 'blood-spirit', reflecting the traditional association of blood with vitality and passion.