* Color and Light Absorption: Darker colors absorb more light than lighter colors. Light is a form of energy, and when it's absorbed, that energy is converted to heat. Black is the darkest color and absorbs the most light, while blonde is a light color and reflects more light.
* Heat Absorption: Since black hair absorbs more light, it also absorbs more heat from the sun (or other heat sources). This can make your scalp and hair feel warmer in sunny conditions.
So, in summary, dyeing your hair black will indeed increase its heat absorption compared to its previous blonde state.