As to runners, you might try pinning them to soil in another pot kept moist. They will probably form roots then leaves (just my best guess...never tried this myself with this plant). As I understand it, there are two black elephant ears, one forms bulb and one forms runners. Both are hardy outside in your zone.

I do bring mine inside in winter just to enjoy, even though hardy outside. The older leaves gradually die of but new smaller leaves appear. I cut back on watering, keeping moist, not saturated. Does well in west window. Canna Pretoria does well here too, along with many tropicals so humidity is probably good. Temps are about 60 F at night, up to 68 F daytime.

As to pot size, they mustn't be overpotted...(too large a pot) but leave 2-3 inches around them to grow. They will need dividing or putting into larger pot next spring. They grow fast with sun & water...I dont use fertilier but some folks do.
Good luck...josh

I have mine growing facing east, so they get full sun in the morning but the rest of the day they just get bright shade. That's enough sun to give them a dark color, but not too much sun.

My favorite way to propagate them is just to divide them.

Anyway, the leaves on Black Magic often come up greenish then darken within a day or so. As to growing inside yearround, leaves won't be as dark (more of a smoky purple) nor will the leaves be as large or tall, and you'd need a very bright window. Perhaps if you divide yours you could try one inside/one outside just to see. The main reason I bring mine in is to get an earlier start in the spring (and to enjoy them all year), but they really prefer outside in summer and more than double in size.

I don't think the bone meal will hurt them if thats the reason you mentioned repotting again. I don't use it but I believe it's a fairly mild growth-enhancer for bulb plants in particular. Just wait a day or so and see if your new leaves turn darker. Good luck....josh

The Black Magic I have are a deep purple, quite lovely. So my point is that they seem to grow better in the Sun down here near the coast.

But... they aren't near as big as the other regular elephant ears I have. I'm hoping this is because last year was their first season in the ground. I bought from Plant Delights which is further North than here so they may be stunted because of the heat. In any event, they didn't burn. I keep them well watered since they are near the back porch. Easy to pour left over water from various kitchen tasks on them. I definitely plan to buy more.

I don't know yet about the runners. Mine did freeze on top during our freeze (only one) this year but it is growning back again now this Spring. Hope it will be bigger and better.