Repeated Bites
One of the reasons a bite from the black mamba is so dangerous is that you rarely get bit only once. The black mamba is the fastest snake that lives on land. Usually, that speed is used for escaping from predators, but when threatened it also comes in handy on the offensive. The snake will strike its victim repeatedly when it attacks, and each bite injects a great deal of poison.
Neurotoxin
The venom of the black mamba contains two different toxins. The first is a neurotoxin that affects the central nervous system in the body. The neurotoxin manifests in the early stages of the bite by making the limbs tingle and degrading muscle control in the body. This shows up first in mild ways, such as dropping eyelids. Later, muscle spasms will begin. Eventually, the neurotoxin in the venom will paralyze someone who was bitten.
Cardiotoxin
The other part of the venom of the black mamba affects the cardiovascular system in the body. Eventually, the venom will start to affect the heart. This manifests as irregular heartbeats after the initial bite and eventually results in heart failure. The amount of time it takes for this to happen depends on things like the amount of venom the snake delivered and the size of the victim.
Lingering Effects
When snakes bite you, the type of lingering effects that you experience depend on the type of toxin that the snake produces. The neurotoxin of the black mamba does not have lingering effects. When other snakes bite you, sometimes you will feel ill for days after the bite. Assuming black mamba antivenin is administered and a person lives through the bite, they should feel back to normal the following day.