From 血(ち) (chi) + 溜(た)まり (tamari, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 溜(た)まる (tamaru).”). The tamari changes to damari as an instance of rendaku (連濁).
ちだまり • (chidamari) ←ちだまり (tidamari)?