Is this a proper past tense?
Today I've darkened 59 appropriate circles.
Strictly "I have darkened" is a perfect tense using the past particple; some people would call it the present perfect. By contrast, "I darkened" would be a past tense; some people would call it the simple past and others the preterite.
Wikipedia has an article on past tenses.
That sentence is grammatically absolutely correct. I’ve darkened is indeed one kind of past tense; specifically, it’s what’s known (depending on how technical one’s being) as the perfect tense or the simple present perfect tense, and also by various other terms. Expanding it to I have darkened, have is an auxiliary verb, and darkened is a past participle.
The sentence is also completely confusing out of context — I have no idea what it’s supposed to mean. Was that deliberate? I’m reminded somewhat of Chomsky’s “colourless green ideas sleep furiously”. Edit: Stuart Bentley in comments points out a very plausible context.