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Planet 9 would certainly be "dim", but whether it would be dark according you your definition is impossible to say.

Planet 9 needs to be smaller or further away than Pluto, otherwise its gravitational effect would have been noticed more clearly by now.  This means it probably reflects less light than Pluto does.  Pluto is too dim to see with the naked eye, but was detected by optical telescopes decades ago.

Whether a dimmer object fits your definition of "dark" can't be judged because the definition is too vague.  You define dark as not visible with a "normal" telescope.  The norm for telescopes, at least leading edge scientific ones, keeps progressing.  There is also huge variation in the light gathering power of scientific telescopes used today.  If you build a big enough mirror and keep other light away from it well enough (like putting it in space), then you can eventually see arbitrarily dim objects.  How big a mirror is "normal"?  3 meters, 10 meters, a much larger space telescope that might be normal before the end of the century?