Dhoni's beard appearing blacker is primarily attributed to reduced stress levels after taking time off from continuous captaincy.
The reference suggests that the intense pressure of constantly leading a team contributed to his beard looking greyer. With a break from the field, his stress lessened, resulting in a more natural appearance, including the perception of a darker beard. While hair color can also be influenced by dyes or other cosmetic treatments, the provided context points specifically to stress reduction as the key factor in this case.
