Blood gum is perfect as a quick vampire costume flourish or straight up gross out prank.
Loup226 is a consistently clever video creator. We're always excited to see his new stuff, but this one is a classic.
Using dried food coloring, hollow out a piece of DoubleBubble and covertly fill it with the red powder. Not only will this freak out the gum chewer, but onlookers will be horrified by the shear amount of oozing 'blood'.
If you really want to sell your gory mouth, a WebMD query lists the following conditions as symptomatic of a bloody mouth: fractured jaw, herpes, HIV, broken tooth, gastrointestinal bleeding, pneumonia, nosebleed.