I have three dogs - two Collies and a mini Aussie. All of them hate having their nails trimmed, especially since I use a Dremel to grind them down after clipping as much as I think is possible without catching the vein. They dislike the sound and the heat on the nail when a Dremel is used. But inevitably, I'll miss and the blood will start flowing, sometimes with the clippers and sometimes with the Dremel, believe it or not. Those black nails can be difficult to judge. Depending on how much into the vein the clipping has gone will determine how difficult it will be to get it to stop.
When the bleeding is at its worst, we have tried everything - stypic pencils usually won't work on dog nails because you need to stick the end of the pencil into the area that is bleeding, and that usually isn't possible. We have tried baking soda as an alternative and if it isn't bleeding too fast, that will work. Sometimes flour will work. We have tried holding ice to the nail, in and out of a baggie, and sometimes that works, as it did today when I got one of the Aussie's nails a bit too close.
But on three occasions with three different dogs over the years, we have had bleeders that simply wouldn't stop, even after holding ice on the nail for over an hour. The first time this happened, I called my vet after 2 hours and asked if there was anything they would do that I hadn't already tried. They told me if nothing else works, use a tourniquet on the leg, leaving it on until the bleeding stops but no longer than 30 minutes, to avoid damage to the leg below the tourniquet. I have found that using that along with ice seems to do the trick when nothing else will. It is important to keep the dog as calm and still as possible when you're trying to get the bleeding to stop. Keep its blood pressure down as much as possible.
I trust you have already got the nail to stop bleeding, but I thought I would share my experience for the next time it happens.