I find the key is to let it soak overnight. I use Vanish, but Sard is also good

I had luck with vanish stain remover powder. Wet the stain and pile a little on top, wet it too. Let it sit overnight.

Find a cleaner specifically designed to break down protein-based stains (look for words like “enzyme” or “protein stain remover”).
Follow the instructions on the product label.
Soak for extended periods (hours or overnight).

Also scrubbing with Solvol block soap can be effective

I’m a fan of scrubbing with Sunlight block soap

Stubborn stains can often take days of repeated soaking and washing for removal.

Intensive and repeated soaking with a preferred stain removal product has worked for us in the past with stubborn stains.

I’ve had success using white vinegar to remove blood stains. For stubborn stains, a short soak. I’m also a fan of vanish spray, but that can fade some fabrics.

Good that you only on cold, that improves the change to remove it as hot water sets the stain. Blood should always be rinsed with cold water.
What colour is the fabric ?
Hydrogen peroxide is an excellent blood stain remover that works well on both fresh and set-in blood stains. However, hydrogen peroxide can have a bleaching effect on some textiles, so it’s important to perform a spot test before using it for stain removal.