At the trial, the defence knew the blood was a match for Robert Boutflour

[Cambridgecutie]

The defence knew at trial that the blood group was a match for Robert Boutflour (and 8% of the population), but it couldn't have been Robert Boutflour's blood for several reasons:

1) The "back-splatter pattern" proved the blood got into the sound moderator from a contact shot.
2) Robert Boutflour would not have known about Sheila's blood group.
3) There was another witness to the finding of the sound moderator
4) There is some other evidence that the sound moderator was on the gun:
      (a) the paint on the end of the moderator
      (b) there was no blood in the barrel of the gun (which would be expected if the moderator was not on the gun)

Re 1) ? Blood stain analysis was in its infancy and not used in this case.  High velocity back spatter from gunshot presents as a fine mist of droplets.  In this we are told a flake measuring a 1/4" presented.

Re 4) (b) Not according to a book written by the late Vincent DiMaio:

"Absence of blood on or in the barrel does not preclude a close range or contact wound.  Thus, no blood was found either on the outside or inside of the barrel in 24% of the suicides using a revolver and 23% using a pistol".

https://en.wikipedia.org/wiki/Vincent_Di_Maio

https://ia801209.us.archive.org/35/items/Gunshot_Wounds_Practical_Aspects_Of_Firearms_Ballistics_Forensic_Techniques/Gunshot_Wounds_Practical_Aspects_Of_Firearms_Ballistics__Forensic_Techniques.pdf

[Cambridgecutie]

Are you claiming that Fletcher didn't know what back-splatter was and how to identify it ??

By his own admission he said it was complicated and not fully appreciated. 

457. Mr Fletcher, the firearms expert, gave evidence to explain how blood got into the moderator if it was attached, or into the barrel if there was no moderator attached. He said that the mechanism was complicated and not then fully appreciated.

Have you read his trial testimony?

The defence needed to bring in experts from the US where gun crime is routine. 

[ILB]

Backspatter is a phenomenon that not all experts agree on.

I am shocked that Nicholas's blood wasn't found in the silencer.

[Cambridgecutie]

Backspatter is a phenomenon that not all experts agree on.

I am shocked that Nicholas's blood wasn't found in the silencer.

Yes especially when you consider it is mainly observed with shots to the head which Nicholas sustained.

[ILB]

Let's be real here.

Why would RWBs blood be in the silencer at his brother in laws house?

It is a weak argument, let's not make bones about it.