The black orb is clearly not part of the riddle. There's only 5 locations in the riddle, and 6 orbs, so the black one is outside of it.
In order:
Delve within the Deepest Dark: Unknown
Kill what lives in Rival's home: TOEE
Look deep where his blessed roam: The Pit
Break the glass in which they slept: WPM
And don't forget where secret's kept: Durk's
The first is probably hiding somewhere in the Darklake. It could also possibly be in the Kingsforest Underdark, but that seems outside of the level range we see on all the others, which are level 9 at max.
Perhaps there's other elements involved, like ooze type. DDO has 6 types of ooze. Grey Ooze, Black Pudding, Ochre Jelly, Violet Slime, Arcane Ooze, and Gelatinous Cube.
Avatar of Juiblex is an arcane ooze.
Muck is a grey ooze.
WPM jars have black puddings.
TOEE is a Gelatinous cube.
That leaves violet slimes and ochre jellies.
Both of those types are found in Depths of Darkness, Discord, and Doom. One of those three seems like the best shot. Of the two types, ochre jelly seems like the more likely candidate, it's a long time old school classic D&D monster.
There aren't a lot of named or notable ochre jellies:
Alluvium is in Whispers of Return - right level range, but in another expansion.
Experimental Byproduct in Soul Survival - too high level.
Festering Sludge in Haunting of Saltmarsh - Different expansion again.
Glomm in Last Stand - Another good candidate, but there doesn't seem to be any delving or darkness connection to it, since this is an outdoor fight.
Phlegm in belly of the beast - Different expansion, level range is off, but it's definitely delving in darkness.