Another commenter said it was the oil on people's hands

Blood oxidizes to a dark brown/black pretty quickly so a red dye wouldn't really hide it.

Yes, but as technique improved, there was less blood introduced on average and the pistachios would not be evenly coloured. To preserve the impression of quality, the farms then adopted the policy of wringing the worst performing worker of each batch of their blood to cover the batch in red homogeneously. This motivated improved performance batch on batch (less blood) and eventually, two workers had to be wrung, and at some point the remaining workers got so good that no blood was introduced and a drastic policy change had to take place.

That or automation.