hematoma, contusion, extravasated
瘀 centers on blood that has leaked from vessels and pooled under the skin or tissue: a bruise, hematoma, or extravasated blood. The idea is stagnant, clotted blood causing discoloration.
瘀 combines 疒 (sickness) with 於, which likely contributes the sound. The character specifically denotes blood stasis or bruising in medical contexts.
The sickness enclosure 疒 wraps around 於, like a bruise forming under the skin. Picture a dark, stagnant pool of blood trapped inside the body.
For ヨ, imagine a bruise that looks like a yo-yo under the skin: yo -> ヨ, and the pooled blood forms a round, discolored mark.