Easy peasy. Use citric acid (available from the supermarket). Sprinkle some on the stain, wet it and rub in. Don’t rub too hard – let the chemical do the work. Dab it dry, and then repeat until it’s all gone.
The chemistry of this is straightforward. The red colour of blood comes from the iron atom contained in the haemoglobin molecule. Citric acid has a very strong affinity for the iron and rips it out of the heart of the haemoglobin, thus disrupting the molecule and destroying the colour.