This is not my blood. It's all splashback from my opponent. I don't have a single wound on my body.
Literal
This [topic-は] my blood [copula-ではない]. All opponent [gen-の] splatter-blood [copula-だ]. My body [to-には] wound one-[counter-つ] not-exist.
返り血 'splashback blood' is a specialized compound (返り 'returning' + 血 'blood') for the blood that spatters back onto the attacker when striking someone. 傷ひとつない '(there is) not a single wound' uses the counter ひとつ + ない for emphatic denial — a dramatic assertion of invulnerability. Three short declarative sentences create a terse, action-movie-like rhythm.