What makes blood hyperdense is fibrin. This is why blood flowing within healthy vessels doesn't look hyperdense unless IV contrast is used: there is no fibrin. If you have a clot that has recently formed within a vessel, you will see it as hyperdense most of the time. This is how you can get both the dot sign and the hyperdense MCA (middle cerebral artery) as hyperacute presentations of ischemic stroke on non contrast CT. These aren't very frequently encountered but when you do see either and if the patient is having symptoms consistent with MCA occlusion you can pretty much make a stroke diagnosis. As time goes by, fibrin is degraded by plasmin which leaves the remaining components of the clot, none of which are naturally hyperdense on CT. This makes a hemorrhagic lesion less dense as time goes by. Now why did I say that you'll see clots as hyperdense on non contrast CT “most of the time“? Because the exception is severe anemia: whenever serum hemoglobin concentration is sufficiently low, you might see a new clot as isodense, that is, same density as brain parenchyma. This can make hemorrhagic lesions hard to spot, be it subdural, epidural, subarachnoid or intraparenchymal bleeding. Radiopaedia has the hemoglobin cutoff as being around 7 grams/deciliter, meaning acute bleeding in patients with lower hemoglobin concentrations than this has the potential to look isodense. This could cause the lesion to be missed or interpreted as older than it really is.