But how can it if the blood could have belonged to a percentage of the population - in every single case?
Are you having a laugh? In itself blood grouping and/or blood type isn't capable of convicting anyone but it can assist implicate or absolve along with other evidence either circumstantial or direct.
Example: Man with ginger hair seen breaking into garden shed around 3am. At 3.15am the police stop a man walking in the vicinity carrying a lawn mower. Man, with ginger hair, says he found lawn mower down an alley and thought it had been dumped so decided to take it home. Police instruct a forensic search of the shed. It appears the perp cut themself breaking in leaving blood on the lock.
A: Blood group and/or type does not match man with ginger hair carrying lawn mower. Useful to defence.
B: Blood group and/or type does match man with ginger hair carrying lawn mower. Useful to prosecution.
Even DNA evidence in itself is not enough for a conviction without other cicumstantial or direct evidence.