Attacking, Journeys in the Dark
I - Rules
(see JITD, p. 19)
If a hero attacks without a weapon, he attacks with his fists. This is a melee attack that grants one red die and has no special ability.
The rulebook doesn't specify it, but a player could logically add a number of black power dice equal to his character's melee trait since Unarmed attacks are considered to be Melee attacks. Fatigue tokens could also be spent to add extra power dice up to the maximum of 5.