He swallowed blood whilst he was praying
You should not have swallowed this blood; you should have taken out a tissue whilst praying and spat the blood into it.
Because you swallowed it without meaning to, your fast is still valid, but if you swallowed it deliberately then you have to make up this day.
Ibn Qudaamah (may Allaah have mercy on him) said: If blood flows in his mouth or he burps up something (reflux) from the stomach, and he swallows it, then he has broken his fast even if it was a small amount, because the basic principle is that anything that goes from the mouth to the stomach breaks the fast.
The basic principle is that the fast is broken by everything that reaches the stomach from the mouth, with exception of saliva because it is not possible to avoid it. But this principle applies to everything else.
And Allaah knows best.