The poem twists a familiar nursery rhyme. "Roses are red, violets are blue" promises bright color and a cheerful, predictable pattern. Here, the colors are stripped away. Roses are black, violets are black, and the whole garden is black too. The repetition makes it feel like a chant, almost like the speaker is going over the same sad facts again and again.
The last line changes everything: "I'm blind." Until then, we might think the garden has died, or that something grim has happened to it. Now we see another possibility. The garden may not have changed at all. The speaker can no longer see it, so everything is dark for them. Judging by the lines, the speaker seems to be mourning the loss of sight and the beauty that came with it.
The mood is quiet, bare, and heavy. The poem is very short, and the plain statements give it a stunned, matter-of-fact tone, as if the speaker is still absorbing what this loss means.