noun
dark red flesh of fish; blood-rich meat near the spine
Refers specifically to the dark, blood-rich meat found along the spine of fish like tuna and bonito. Often contrasted with the lighter, fatty flesh.
マグロの血合いは鉄分が豊富だ。
The dark red flesh of tuna is rich in iron.
血合いの部分は独特の風味がある。
The blood-rich meat has a distinctive flavor.
赤身 is the lean, red meat of fish (like tuna), which may include 血合い but is a broader term. 血合い specifically refers to the darkest, bloodiest part near the spine.
Compound of 血 (chi, 'blood') and 合い (ai, 'joining' or 'meeting'), referring to the area where blood concentrates along the spine.