Comments posted to our Bloodborne Wiki
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I don't know what causes it to trigger but the blood rock keeps popping back up in my game with the insight bath messengers I've bought a few of them all in the same playthrough
As mentioned in the article, Blood Rocks are permanently for sale at the Insight Shop after beating Mergo's Wet Nurse.
Moar Blood Rocks! Rotted Isz, Glyph: xsewnsuu

In the bonus area before 1st boss, not much resistance along the way. Just stay on the top floor.