Board Game Arena

What are the black dice for that show up next to planes area on game board?

They are used to add traffic planes.
Here is from the rule book:

If there is a Traffic icon in the Current Position screen
at the beginning of the round, roll the Traffic die as many
times as there are icons on the space. Each time you
roll, add an Airplane token to the space indicated by
the value of the die, starting with the Current Position
screen. Don’t roll the die or add Airplane tokens if there
is no Traffic icon in the Current Position screen, even if
you have passed through one when advancing 2 spaces.

As per the above description, the black dice indicate what values were rolled for traffic and where additional planes were placed on the approach.