You might be able to see that the black baby is slightly smaller than the other. But is it a Peanut or a Runt?

Peanuts

Peanuts come from a mating of two true dwarfs, which each carry a true dwarf gene. Some kits in the litter will have this gene which is fatal. Survivors typically last only a few days, though there are some accounts that have survivors up to six weeks of age. They are very small, typically only 1/3 the size of their litter mates. These kits are typically absorbed in uterus, accounting for the small size of the dwarf breed litters. If you do end up with a peanut, you know without a doubt, that both the parents are true dwarfs.

Things to look for in a peanut

A small undernourished kit

Delayed development

Poorly developed hindquarters

Peanut shape

Paper thin ears

Bulging eyes

Large head in proportion to the rest of their body

Inability to thrive

Death within a week of birth

Runts

Runts can occur in any breed and any litter. They are typically smaller than their siblings, but they can grow up and live a normal life. You might find them out of the nest box , as they often hang on to their mom for too long. They normally happen when a kit is deprived of nutrients either in or out of the womb.
Things to look for in a runt.

Smaller than siblings

Proportional body to head

Will to survive

What you can do for a runt.

Feed mom alfalfa hay. This provides essential nutrients for any baby bunny

One on one feedings with the mother

Keep runt with mom for longer than litter mates.