Hello, it looks like this rose may be a goner sorry to say. But a few quick tips that may help in the future.
1)If a rose has black spot you can pull off all of the leafs and discard them if it is a extreme case or treat with chemicals. But it looks like you trimmed stems back, if this is the case dont anymore. This takes valuable energy from the plant that is already stessed, it may have sent this poor plant over the edge.

2)If you continue to grow roses in pots, DO NOT LEAVE THE SAUCER ON THE BOTTOM OF THE POT!!!!! This is the biggest killer of a potted rose. The roots do not like to sit in water. With an established rose growing nicely you want to water when the top of the soil looks dry and the pot feels light when you pick it up. Then water deeply with lots of water and no saucer on bottom. Do not let the pot dry out and the leaves wilt and drop, it can be somewhat of a tricky balancing act. You may find you have to water a couple times a day with very high temps.

3) Make sure to use a good potting soil and add some perlite for good drainage, and sun sun sun and more sun you cannot give a rose to much sun, but you may have to provide afternoon shade in high temps. the sun and high temps will also help with the blackspot and mildew.
good luck bob

All of Bob's advice is sound. You do have one good green cane there though so don't give up hope. Most minis are own root and it should send up new canes. Cut off the brown/black canes and don't give it any fertilizer until it leafs back out or you see new growth coming up. Keep it watered and in the sun and it should be OK.