black (horse)
驖 centers on a black horse, specifically a horse with a deep black coat. The meaning is narrow and concrete, with no clear abstract extensions.
驖 combines 馬 (horse) with 鐵 (iron), likely suggesting a horse with an iron-black coat. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The left side is a horse 馬, and the right side looks like a dense, metallic structure. Imagine a horse with a coat so dark it gleams like black iron.
For テツ, picture a black horse named Tetsu, its coat shining like iron: Tetsu -> テツ, and the horse is as dark as metal.