If the fasting person detects the taste of blood in his throat, that does not affect him even if he swallows it, but if it comes up to the mouth then he swallows it, he breaks his fast.
The same is said concerning phlegm, sputum and everything that may appear in the throat.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: I should point out with regard to the issue of sputum and phlegm, because some fasting people make it too hard for themselves, and if they notice that in the deepest part of their throats, they try very hard to expel it, and this is a mistake, because phlegm and sputum do not spoil a person’s fast unless they reach his mouth then he swallows them, in which case it breaks the fast according to some scholars, but according to others that does not break the fast either.
But that which is in his throat and goes down into his stomach does not break his fast, even if he feels it, so he should not tire himself out by trying to expel that which is in his throat of these things. End quote from Fataawa al-Shaykh Ibn ‘Uthaymeen (19/356).