While he's not initially described as "dark," he's initially presented as a mysterious figure:
* He's new to the area. This makes him a potential target and a subject of suspicion.
* He's tall and imposing. His physical presence contributes to an aura of mystery.
* He's shrouded in a cloak. This further obscures his appearance and makes him appear shadowy.
It's important to remember that Sherlock Holmes and Dr. Watson are initially unaware of Sir Henry's true identity. They see him as a "stranger" on the moor, which fuels their investigation into the legend of the hound and the potential threat to Sir Henry's life.