According to AE it was a blob of jelly, like jam and there was a white hair attached, thik he may have noticed this but heyho maybe she made a mistake. 
She described the blood inside the hole as match stick head sized. The blood outside of it was even smaller that is why the lab was unable to successfully type test it.
They didn't notice the blood until they very carefully studied it at Oak Farm. Why would Jeremy carefully study it when he didn't appreciate the significance the blood could have? If it had a significant amount of blood on the outside that you would clean just because a ton of blood would be obvious to be related to the killings.
Why would he care about a hair? How do you know the hair wasn't in the closet and didn't get attached to it after being put in the closet? I am sure the house and closets had Nevill's hair around I don't see how a hair means squat it could get attached before, during or after the murder. If it was Jeremy's gun located at Jeremy's house and had Nevill's hair on it that would be more significant.