SBurt, did you use potting soil in your pot or regular garden soil? What type of fertilizer did you use? Have you used any sprays, e.g., for fungus, bugs, etc.?
Your leaves show signs of chlorosis, (green veins and yellow leaves). That could be an iron deficiency. Looking at your meter (hopefully, it is accurate), it is showing that the soil in your pot is alkaline. The high Ph could be locking up the iron in the soil and not making it available to the plant. Roses prefer a Ph of 6.0--6.5.
The canes turning black could be a sign of canker. Canker is a fungal disease and you need to prune those canes out at least 2 to 3 inches below the canker, to a strongly growing shoot/cane. You also need to make sure you wipe off the blades of your clippers with alcohol, so you don't transfer the fungus to a healthy part of the plant.
I grow roses in pots and have for decades. Make sure your pot is big enough for the rose and that it has good root depth. I use pots that are at least 24 inches in diameter and at least 24 inches (or more) in depth. I also use a good potting soil as well so that they plant will have adequate drainage. The pots need to have several holes in them for drainage. I prefer resin pots for my roses for several reasons rather than ceramic (heavy, can crack and the expense). Make sure your pot is not sitting in a saucer where the roots can get waterlogged.
Roses in pots will need to be fed more often. I use a good organic fertilizer for this. How often did you fertilize? Over fertilization can cause burning of your rose. This is one time when more is not necessarily better.
