If you didn't notice this after you posted, scroll up where the last three questions before yours in this topic are also where to find the blood, and you will see the answer (a hint, the room location, and a spoiler with exact details).

Specifically, look on the table/bed across from the Amdusias' chest (that asks for a password) for the "small vial, that looks kind of like a candle at first glance...". There just isn't that much in the section of the room that was hidden, so look at everything.

Welcome to the forum. wave