Yes Adam all anyone needed to know was that a silencer in the cupboard with Sheila's blood in it would convict JB and notice I said anyone.
I think you mean Sheila's blood groupings shared by about 8% of the white British population.
At a minimum any 'planter' would need to know Sheila's groupings and have access to blood with those groupings.
Forget David's diluted period blood theory because I can assure you 100% that conventional serological analysis of blood, used in this case, requires good quality dried blood of a certain quantity.