Blue flames

“I’ll protect you,” he murmured. He clenched his fist tightly.
The physical arts of the ninja were not about strength. He might have been
a small child of five years, body as yet undeveloped, but as long as he
managed to knead his chakra properly, he could smash even this enormous
rock.
He brandished his fist high in the air. Chakra filled his arm, covering it in
faint blue flames.
The rock slammed into Itachi’s fist, and exploded with a shriek.
Even a child could effortlessly destroy boulders, as long as they simply
kept up their ninja training.
Showered in a rain of pebbles, he landed soundlessly on the ground.

Itachi Shinden: Book of Bright Light Chapter 1 Part 2