The blood is actually about the grease. The more you use a knife to slice up animals, the more blood and fat get onto the blade, coating it and making it duller. That's why when you butcher animals, you need to wash your blade with hot water to keep it sharp. You can still kill with a fat dulled blade, but it can cause a slice to falter at a critical moment. Clearly, Goblin Slayer isn't the type to risk that.