The speaker is puzzled and a little disgusted by a contradiction: ravens look glamorous, "black velvet," bright and shiny, yet they feed on "the worst blood" among the pebbles. The question that opens the poem, "How's possible?", sounds like genuine disbelief that something so handsome could behave so ugly.
In the second stanza the bird admires its own reflection in dirty water, while its ego whispers about the next victim. That makes the raven a figure of vanity as well as hunger. Then the view widens: a black stain across the sky, ravens at twilight, dancing in someone's eyes. The speaker imagines a "flying nightmare" that bites the worst. The ending admits the trap: the ravens "lied" because they "dressed up damn nice." Appearance fooled the speaker.
The mood is dark and uneasy, with a hint of bitter wisdom. The poem seems to warn that polish and charm can hide something predatory, and the ravens may stand for people as much as birds.