Is the fast valid if a fasting person practices masturbation during the day in Ramadan and prevents the emission of semen, resulting in blood coming out instead?

The drop that came out of you is semen, and its color may change to red or bloody. Al-Nawawi said that semen may turn red and become like meat water, or come out as fresh blood, and it remains pure and necessitates ghusl (definition).

Therefore, your fast has been invalidated, and you must make it up and repent from intentionally masturbating. No expiation is required of you, and if you have not performed ghusl, you must repeat the prayers you performed before ghusl.