If I visit a blog that writes about what cats dream about, and I write a comment in the comments' section; "There's also a difference between the dreams of a cat who doesn't go outside and a cat who does, and what a cat dreams about depends on the cat's personality. I wrote an article about it." And then I leave a link to my post about feline dreams. Is leaving a link under the circumstances considered spam?
Leaving one relevant comment with a link shouldn't be considered as spam. One the other hand, since you'll be "stealing" their visitors, they may deem it as spam.
It depends actually, some may consider it as spam, others may not, but generally, I believe leaving a comment with a link shouldn't be considered as spam, man it's omnipresent it's everywhere!
It comes off as spammy to me. If you're commenting on a blog for the sole purpose of linking back to your website it's going to be hard not to come off as spamming in my personal opinion.