You are either being disingenous or suffer profound learning dissabilities. The flake of blood underpinning JB's conviction was based on far more than ABO groupings; it was based on gel electrophoresis to identify enzyme and protein groups. Gel electrophoresis requires good quality and quantity of blood to produce results:
ABO PGM EAP AK Hp
Nevill Bamber O PGM1+ EAP BA AK1 Hp2-1
June Bamber A PGM1+ EAP BA AK2-1 Hp2-1
Daniel Caffell O PGM2+1+ EAP B AK1 Hp2
Nicholas Caffell O PGM2+1+ EAP B AK1 Hp2
Sheila Caffell A PGM1+ EAP BA AK1 Hp2-1
From the 2002 CoA:
482. No questions were asked at trial of Mr Hayward to establish what part of the blood he
had tested. The position was, however, known to the defence through their own
expert Dr Lincoln. Dr Lincoln had seen the evidential material upon which the group
testing results were based and agreed with the conclusions. He recorded that evidence
in the course of his report of 19 September 1986. He said that Mr Hayward had
“found a flake of blood trapped under the first or second baffle plate” and that it was
this flake that was tested and produced the groupings A, EAP BA, AK1, Hp2.1 upon
which reliance was placed by the prosecution. Dr Lincoln further recorded:
“Mr Hayward states that he could detect visible staining on the
“upper baffle plates” and that he swabbed these plates so that
the blood was taken onto cotton material which could
subsequently be used in grouping tests. On this material Mr
Hayward successfully determined the ABO and EAP groups
and showed the blood to be groups A, EAP BA.”
https://ceadstorage.blob.core.windows.net/cead-images/Bamber.PDF