If a blackbird landed on my shoulder, I'd imagine that bird thought it knew me. I'd assume it was an old friend of mine or an ancestor trying to communicate or reach out for some reason. I'd ask myself which friend/family member was trying to connect with me, and I'd look at the details of the situation to try to guess who (and imagine what, exactly, they were trying to convey). Where you were and what you were doing and what you were thinking about right before the bird landed. Those are all important parts of the story. I'd also take it as a sign of my relationship with and connection to the environment/world/universe.