So I did eventually realise that you could put concrete on the fat tiles... It did however remind me of something I remembered reading a good while ago which I finally found. There was a research paper called "StarCrete: A starch-based biocomposite for off-world construction" I checked and it's the top result if you google that. They were trying to come up with ideas for concrete like substances in space. Anyway there is a reference to a team that produced a concrete like substance from serum albumin (which is a protein found in blood), though I suspect they probably didn't call it bloodcrete. For factorio purposes you could make something similar which is just the vanilla concrete and foundry recipes with water replaced with blood. You could just use a red tinted concrete for the design.