So what does blurred vision have to do with the crucifixion wounds of Jesus?
It starts with the branding of slaves with a tattoo. This mark was known as a stigma, from stig, to prick (tattoos are created by pricking the skin with ink-filled needles). The term stigma was then generalised to apply to the slaves themselves, and thence to a distinguishing mark of social disgrace, such as 'the stigma of being in prison'. From a similar origin came the meaning of any mark on the skin, and of the marks of crucifixion in particular.
Back on a different track, but to the same source, the blurred vision of astigmatism is caused by an oddly-shaped eyeball preventing light rays from merging at a single point on the retina. Thus we have a + stigma, where 'a' means 'not', and stigma is from the greek for 'point' (and 'prick').
Incidentally, stigmata and stigmas are both acceptable plurals for stigma.