What if a black dwarf entered the solar system?

The odds are less than short.

why are you asking.. who cares  what if santa claus is real

a black dwarf is a  sun that  became a white dwarf then cooled down so its a NON SOLAR  mass 

if a LARGE mass were to travel through our solar system it would   MOVE OBJECTS BY gravity

If it came from outer space, it would soon enough go back again.

Its effects would directly depend on its mass and its trajectory.

Without knowing either, we can't say what would happen.

It would make a great April 1st Joke

Watch Family Guy

If something like that happened, we would know about it a VERY long time in advance.

When it arrived, it would cause a lot of disruption. Just how much and where, we could not know until we could find out its mass and predict its exact course.

The Universe isn't thought to be old enough for any black dwarfs to have come into existence. It would involve a white dwarf cooling to below about 700 Kelvin, which has never happened so far since the oldest white dwarfs would only be thirteen thousand million years old at this point. Once they have formed, the Sun will have destroyed many of the planets in this solar system, so the situation will not arise.

So far, the universe isn't old enough for this to happen.  It will take many, many billions of years for a white dwarf to fade to black...