I KNOW there are others that can answer this better, but I'm troubled that you seem to be saying you took the soil-rooted cutting out of the all-potting soil and put it in WATER ??
Roses need well-drained soil and if you used store-bought potting soil, it's very possible that it was too heavy. The original 50/50 sand and potting soil was probably the better mix, especially if it was happy in that. But I'm sure rose won't survive in water. I don't even think you can root rose cutting in water. And even if it did, water-grown roots are different from soil-grown roots and vice versa.
You said TOP half of the cutting was black and you cut it off. Right? Are there any buds left in the remaining, presumably rooted portion of the cutting? If there are more buds to grow from, it might have a chance. Does the cut look clean? No black/brown layer anywhere? If there are, I think you need to cut down further.
If the blackness resulted from disease, then I'm not sure if the cutting will survive, but at the very least, I think the rooted cutting should be potted up in a sterile soil mix. I'd want to treat the cut end with some kind of a fungicide... but being a strictly organic grower I can't think of what I'd use in this case -- definitely some milk and water, maybe cinnamon or chamomile tea, or Aerated Compost Tea, maybe a spritz of alcohol....
If roots were filling the original container, was there some top growth... or is it dormant now? Shouldn't it be dormant now if you're in Virginia? But you said you "recently" rooted it? Hm... I'm a bit confused
Oh, wait, you've been growing this cutting for the past 4 months? Where (what kind of environment) are/were you keeping the rooted cutting?