https://simplyforensic.com/how-luminol-reveals-hidden-blood/#google_vignette 

Make your own: mix cornflour, corn syrup, water and of course, red food colouring. You can also add chocolate syrup for a thicker texture and darker appearance, perfect for creating drips of fake blood.

Fake Blood Reactive to Luminol – (Iron Tablet Method) 

Materials Needed 

• Corn syrup (or glucose syrup)
• Red food colouring
• Cocoa powder (optional, for realistic colour)
• 1 iron supplement tablet (e.g., ferrous sulfate)
• Water
• Luminol test solution (commercial forensic kit)