Welcome to BYC, Rancheletti.
I am sorry for your loss... it's never easy, but especially when they go so suddenly that we don't even get a chance to help them.
Seizures can be caused by a number of things... Injury, age, toxins, poison, ingesting lead shot/paint, moldy/rancid feed, congenital defects... Without a necropsy, there really is no way to know.
Would you consider performing a necropsy on your bird? Particularly the crop and gizzard? If he swallowed a sharp piece of metal, that could explain seizures and blood in the mouth.
If you have another bird experience these symptoms, I would place its body in the refrigerator wrapped in plastic bags and contact your local vet for information on where to send for your state's USDA office to perform a professional necropsy. It's free, outside of shipping costs (maybe $6ish?), and you will receive a copy of the results, so you can have an idea of what to treat for (if anything) and have a plan for your flock moving forward.
Again, I am so sorry your rooster passed this way. Hopefully, it will be a one-time occurrence.
MrsB