Mystery solved! To everyone except native Lilliwaupians the blood oyster is known as a rock oyster, or jingle shell. Officially, the animal is called Pododesmus cepio. Other names include Pacific jingle, blister shell, false jingle shell, green false jingle shell, and money shell. Another official name is Pododesmus macrochisma. The bivalve lives in the low intertidal zone to 300 feet. It's not a scallop (but thank-you to the people wrote in suggesting that the red meat was actually scallop roe). And it's not a clam. But it isn't really an oyster, either. Here's a photo of a live one:
'Jingle shell' is used to describe various species of bivalve that extend byssal threads through their bottom shells to attach themselves to rocks (or pilings, or other shells, etc). The delicate, translucent shells make a jingling sound when strung together in wind chimes or necklaces.