The blood samples from SC didn't go straight to the lab. There were three samples taken and only two were used. The samples were kept in the SOCO fridge and as we know, there is no mention of the silencer until September.
The 3 samples were transferred to the lab though.
The samples had seals on the bottles and also a bag that contained the bottles. The lab found no evidence the seals had bene broken. You therefore have no evidence that the police planted anything.
Indeed the police considered the significance of the suppressor to be the potential for prints.
Any planting of evidence would have to have been done by the lab which not only needed to plany evidence but remove evidence because if the gun had been used without the suppressor then the gun itself would have had blood in it which needed to vanish.
There of course is no evidence of any of this happening. You need proof to discount the suppressor evidence and have none.