First, an important point to clarify, according to the Torah, every single human
on earth, regardless of skin colour, descended from Adam and Chava (Eve),
not just the Israelites. The Torah does not specify the skin colour of Adam and
Chava so it is pointless to guess what colour they might have been.
Second, there is no basis for the claim that the Israelites were black as they
did not come from Africa but rather, from what is referred to as the Middle East
today. The original core group of Israelites would have had an olive skin tone.
Neither the Tanach (Jewish Bible) or science supports the idea that the Israelites
were black.
And if you have trouble with this answer and still need to believe that the
"Israelites" were black, then there's a simple way to settle all of the disturbing
uncertainty in your question. All you have to do is relax your mind and accept
the fact that Adam and Eve were black.
Answer:
It is because Adam and Eve were black, themselves, so there wasn't a
difference in color between them and their more distant descendants, through Abraham, Isaac, and Jacob, the Israelites.