According to Dictionary.com:
sludge –noun
1. mud, mire, or ooze; slush.
2. a deposit of ooze at the bottom of a body of water.
3. any of various more or less mudlike deposits or mixtures.
4. the sediment in a steam boiler or water tank.
5. broken ice, as on the sea.
6. a mixture of some finely powdered substance and water.
7. sediment deposited during the treatment of sewage.
8. Also called activated sludge. Bacteriology. sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.
9. a fine, mudlike powder produced by a mining drill.
(Based on the Random House Unabridged Dictionary.)
However, Princeton University defines as such:
sludge -noun
1. the precipitate produced by sewage treatment
2. any thick messy substance
So thus it appears, Sludge can indeed take many shapes and forms.
And a Sludge by any other name .. wait nevermind, lol. Maybe it's a British thing.
EDIT: I personally always wondered why a Pile of Sludge is black (and "pointy" as you say) and a Bucket of Sludge is white (and "silty"), though.
Maybe it is industrial waste from Sophie's Lab.
Is this the right thread to also wonder why we have toilets in Neopia but no toilet paper?