If Y)R means the Nile River in Egypt, then the context is strongly suggesting
that $XR at Isaiah 23: 3 is referring to Egypt.
I write:
What this parallelism with Y)R connotes is precisely what Egyptian
documents show us, namely that Shi-Hor (Water-body of Horus) was the
name for a connected body of water comprising a bay and the
easternmost branch of the Nile, later known as the Pelusiac branch.
This was considered by the Egyptians to be the eastern border of
their land. The Shi-Hor and its surrounding area has been popularly
well-described by James Hoffmeier in his two books _Israel in Egypt_
and _Ancient Israel in Sinai_.
The Nile Delta was not as extensive as now in the mid to late second
millennium and the Shi-Hor was particularly the name of a bay off the
Mediterranean sea at that time, but also to the branch of the Nile
that emptied into that bay or very near it into the Mediterranean.
This bay is directly north of Tell Hebua, the site of Silu/Tjaru as
proven by the excavations of Mohammed Abd el-Maksoud. In either the
late second or early first millennium the delta extended to the point
where the bay was cut off from the Mediterranean and eventually dried
up. It's still visible in satellite photos as a large depression in
the area. From that point onward, the easternmost branch of the Nile
retained the name Shi-Hor.
There is absolutely no doubt about Shi-Hor being this body of water.
A coincidental similarity with a Hebrew root for black is only that:
coincidental.
You should rather find a reflection of the Egyptian word for their
"Black Land" (referring to Lower Egypt) in the Hebrew XM. As is
otherwise well-known, by the New Kingdom, the feminine marker final -T
had dropped out of pronunciation, though it was retained in writing.