Actually pretty common with red wing blackbirds. Back when I was doing a lot of nature photography I was hit a couple of times. Always by surprise though, once I knew a bird was protecting its territory I wouldn't stay too close to stress it. Best was to watch and find out where their favorite perch was and just focus on it and wait for them.
Used to to get dive-bombed in the spring walking to my car in the East lot of long term parking at DIA. Birds made nests in the lot signs and then protected them aggressively from us weary business travelers.
I used to love the Red Wing Blackbirds in the Denver Tech Center when I worked down there. A number of swampy areas between buildings and such, which they love. Never got attacked, but I loved their trilling call. Most blackbirds sound like a rusty gate, but not the Red Wing. Saw them many times perched on cattails and such.