I would re-pot it and then you can see what the roots looks like, sounds like the soil way have been to wet.

Your Geranium has blackleg --a fatal Geranium disease, and there is no hope for it. It may have had it when you bought it. It takes a while to get to the state you are describing. It is usually caused by overwatering. Geraniums like to be kept on the dry side.

I guess it's a goner. I'll report this to the vendor. Guess I should have done an inspection of the lower stems!

Thanks oilpainter for diagnosing what ails it.

I see this is an older message so any advice would be useless at this point. However, you might have been able to save this plant had you cut it near a note, trimmed off any flowers and planted it so see if it would root, as we do in propagating pelargoniums.

I hope you were able to replace.

Nancy

Don't give up on it; yes black leaves usually says you've given it too much, but try to save what stems you can....and start new plants.
Any black around the roots does suggest you've lost it...bacteria is probably rampant and to save it would probably be a waste of time but, sometimes if you can split the roots, and give to the new planting some "Damp Off"...a fungicide, a couple of drops in the first watering, it might react and come along.
Anyway, with geraniums you never can tell, they'll come back from a winter's dryness if you don't try to push them to produce.

You can't save a plant with blackleg. I have tried taking cuttings from such a plant and they died too. A plant with blackleg is a gonner and there is no way around it.