I had a coach ask me if I had heard of this. He said it's purpose is it can be sprayed on the player's jersey where there is blood and it will essentially make it safe for them to continue playing. He was told that NFHS approved this but I don't know his source (probably salesperson). If anyone has heard of it please let me know so that I can call him and tell him. I'll probably just refer him to the State Association's office.

Cavicide is a surface disinfectant used for cleaning in hospitals - usually it's applied to hard, non porous surfaces and instruments. I'm not sure how it works on clothing. Visit metrex.com for more details but ultimately it is up to the school administrators if this is an effective and safe alternative to changing the jersey.

I'd say if the jersey's still wet from being soaked in blood, or very red from being soaked in blood, it can't be worn. Regardless of anything that gets sprayed on it.

Once again I will remind people that a player CAN participate with blood on the uniform. 3-3-6 prevents a player who has an EXCESSIVE amount of blood on his/her uniform from playing. It is up to the official to determine what is excessive. So, just because there is a spot of blood on the uniform, you don't have to send the player out.

What's wrong with giving the guy a new shirt, or a shirt from one of the benchwarmers?? In our association we are told not to penalize switiching numbers for this reason, because neither team had any control over it.

There really shouldn't have been any need for your association to tell you that. There was already a rule in place to cover this situation. Case book play 3.3.6SitA.

There's absolutely nothing wrong with that. In fact, the Case Book specifically says that you may do this without the penalty of a technical foul in 3.3.6 Sit A.
However, remember 1: that the benchwarmer you take the shirt from must not have played already and now may not play unless you can find him another shirt with an unused number because a team is never allowed to have identical numbers on two different players.
And 2: if you do make the kid switch jerseys, he DOES have to leave the game at that point or his team must take a time-out.
All I am saying is that the official does have the option to allow the kid to remain in the game with his original uniform, if in that official's judgment the amount of blood is not excessive. Most officials simply don't realize this. They see blood and automatically make the player leave.
Therefore, I simply point this out as a good thing to keep in mind. You never know when you will have a team from a small school on the road, who is short of both jerseys and players. I think it is much easier AND ALSO PERMISSIBLE to just let the kid play in most circumstances.

Dang, JR, if you are half as fast on the court... :)