Tweak portions as necessary - getting the right consistency can take a few batches.
1/2 cup flavorless gelatin powder
1-1/2 cup water, or whatever is just a little more than the gelatin recipe suggests
1 tsp caramel food coloring
1 cup darkish red dish soap
Mix it up, microwave for short intervals until it all melts together.
I found, when I was mixing the stuff nightly, that letting the gelatin set overnight made it easier - I could put the rest of the ingredients together while it loosened in the microwave, and then put it all together. This reduced the risk of boiling the blood - and making the soap froth up!
Used this for Harold and Maude, for the scene where Harold stabs himself to get away from Date #3. He wore a white-and-blue striped shirt - and after the laundry was done, we couldn't tell that it had ever seen "blood."
Hope this helps!!
Meg