First, you need a black hole…
Luckily, Dungeons and Dragons 3.5 has your back. Elder Evils in the EL 20-21 encounter "The Obelisk" describes a caster who, under unusual circumstances, combines a sphere of annihilation with a well of many worlds. Five rounds after he does the
sphere becomes a "black hole," annihilating everything in existence. If abandoned, it picks up speed and power, drawing the island, the waters, the air and everything else on the Material Plane into its dimensional fissure, erasing all existence in 1d12+6 minutes. (142)
The real fun begins on the next page's Features of the Area under Sphere of Annihilation:
Once the ["black hole" is created], the resulting tear pulls air, fine creatures, and debris toward it. After the first minute, the effect intensifies, and all characters must succeed on DC 15 Balance checks. A failed check means the character falls prone. A check failed by 5 or more indicates the character falls prone and slides 1d2×5 feet toward the sphere. Each minute thereafter, the Balance DC increases by 5. After 3 minutes, flying anywhere within 100 feet is impossible, and such characters are automatically sucked into the sphere unless they succeed on a Reflex save against the Balance check DC. (143)
Yes, resisting the black hole's pull requires increasingly difficult Balance skill checks. No one's laughing at the thief-acrobat now, are they?
Apparently, there's no range at which a creature doesn't make these Balance skill checks. Every creature on the Material Plane makes a Balance skill check, falls prone if it fails, and slides toward the black hole if it fails by 5 or more. Which is hilarious.
(I think, in the initial description, the author assumes the destruction of the Material Plane (and, by extension, all creatures thereon) because even level 21 characters will struggle to succeed on the, like, DC 100 Balance skill checks needed to resist the black hole's pull, but players are infinitely resourceful, so I wouldn't put it past them to survive beyond the given 18 minutes, especially if they manage to employ a greater teleport, which has "no range limit.")
While the Elder Evils black hole is inside quotation marks—likely indicating it's not a real hard-science astronomical feature but a fantasy black hole—, unless you want actual science in a fantasy game (brr), these rules seem a playable enough.
…And that may (or may not) mean a doomed umbral blot
Thus, in the abstract, an umbral blot (EL 223-4) will be no match for this ersatz black hole. While the umbral blot has a Balance skill check bonus of +10 due to its Dexterity, it will eventually hit the sphere, and the umbral blot will be destroyed. Although described as a "hovering sphere of absolute void," the umbral blot is ultimately merely another construct, therefore vulnerable to a sphere of annihilation.
This shouldn't happen to the umbral blot, though. The creature can use both greater teleport and plane shift as extraordinary abilities at will. But, were it somehow convinced to travel within 100 ft. of the black hole, the umbral blot will be consumed like anything else. The umbral blot's greatest weakness? It has only a fly speed and flying creatures "are automatically sucked into the sphere unless they succeed on a Reflex save against the Balance check DC." Who knew that would doom a creature with a challenge rating of 32?