Adult black mambas range from olive and yellow-brown through grey to a dark gunmetal shade, and juveniles are often paler still. The colour that gives the species its name is the mucous membrane of the mouth, which is inky and is displayed when the snake gapes, spreads a narrow neck flange and rears in a defensive threat.
That display is the snake's preferred response. The species is fast for a snake and has been credibly clocked at something over ten kilometres per hour in short bursts across open ground, but that speed is used to reach cover, not to chase people. Popular claims of mambas pursuing and outrunning humans do not survive scrutiny.
What is genuinely serious is the venom, a neurotoxin-rich mixture including dendrotoxins that can produce rapid paralysis, and a bite is a medical emergency requiring antivenom. Precise historical fatality rates for untreated bites are quoted with more confidence than the underlying case records support.