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If the temps have dipped down pretty low at night at any time, it could be a bit of frost burn on the petal edges. Also, some of the darker red roses will at times have nearly black edges on their buds. However, if they are a bit crispy, as well as being black, it is either frost or wind burn since the heat is not an issue. It could also be a fungus, but most likely it is not that or the entire bud would have problems.

Your best shot is to buy growth hormone, wet the shoot that will be the root
place it in soil that is for growing seeds, cover with a plastic (spray water lightly first) and out it in your kitchen window.
I have rooted many plants just by sticking it in a huge pot I have next to my front door, it gets sun buts is sheltered. Also I have rooted tomato clippings
begonia, onions lettuce, spinach, LOL just by putting the root in a bit of water.
After they develop root I set it in a small pot
By the way where did you get a thornless rose, I have lymphadema and cannot have rose.

Well I have had the Bayer products cause this if they are spray applied on white roses. Also if the temps are in the 90's or 100's where you are, the rosebush just simply cannot get enough moisture to the petal edges in the heat to protect them and keep them moisturized, thus they burn or turn brown, crispy edged. Keep an eye on the soils moisture and rinse the entire rosebush down with a watering wand in the early evening once the temps get cooler, this will help if it is heat related as well.

Stan the Roseman
Consulting Rosarian

I think these might be called suckers. They are rough stem that take all the goodness, hence there long size and do not give flowers. My mother always tells me to cut these offf at the base and don't let or encourage there growth. MY INFO IS ONLY VASED ON MY PERSONAL EXPERIENCE AND MUMS ADVICE. gOOD LUCK

The brown edges on the petals can be caused by a few different things. Sometimes it is just the age of the bloom and the edges of the petals will start to turn brown as the bloom ages and dies. If the blooms are new blooms, it could be a bit of light frost or cold damage if you have had some cold nights. The rose bush just cannot get enough moisture to the edges of the petals to protect them from being bitten by the cold and frost. It can also be caused by windy conditions that cause a bit of wind burn to the edges of the petals for the same lack of moisture to the delicate petal edges as mentioned previously. Also, if the temps have been very hot where you are, mid 90's to 100's, the petal edges can get a bit cooked. Again, this is due to the inability to get enough moisture to the edges of the delicate petals. Sometimes it can be caused by a fungus or insect problem, but these will usually show distress to more than just the petal edges, the entire bud or bloom will be affected. If the brown edges are a bit crispy, then the wind, heat or cold is usually the problem leading to the brown edges.

Give neem oil a try. It is a good, organic pesticide as well as a fungicide. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm