A blemish is a stain, a mark of a different color that marks dirt. In the figurative sense represents something shameful, which has a negative effect on the person's reputation.
As for example in the sentence, "no blemish was found in his career, he had an unblemished character".
Learn more about the meaning of Unblemished.
To have a blemish is to have a defect, so something immaculate is what is perfect, with no snags or anything negative to point out.
Medicine uses the word macula to designate more than one thing, whether pathologies or parts of the human body.
In dermatology, the macula is a type of skin lesion caused by sun exposure, signs of aging, burns, among other factors.
In ophthalmology, the macula is a part of the retina of the human eye that differs precisely because it resembles a spot at the back of the eye. One of its functions is to allow the view of details, and macula malfunction can be diagnosed while macular degeneration, something common as age advances. One macular edema it's a swelling of the retina that can impair vision, and it's a diabetes-related illness.
And there is also the macula dense, part of the excretory system, being an element of the juxtaglomerular apparatus.