sea of blood; pool of blood
Used literally for a large amount of blood covering an area, often in violent or accident scenes. Can also be used figuratively for a scene of great carnage.
事故現場は血の海だった。
The accident scene was a sea of blood.
戦場はまさに血の海と化した。
The battlefield literally turned into a sea of blood.
Compound of 血 (chi, 'blood') + の (no, possessive particle) + 海 (umi, 'sea'). The phrase is straightforward and its meaning is transparent from its components.