The usual (more-or-less internationally accepted) way to do a blockade is to intercept ships of a neutral flag and order them to turn around. Just shooting them with anti-ship missiles would be totally out-of-line and upset China, so I'm not surprised that Russia didn't do that.

Even if they did, I don't think the next rung on the escalation ladder is just shooting at the RU ship ourselves. If it were me in charge (very non-credible take incoming), I'd probably have US-gov-insured ships sail under Turkish flags with Turkish escorts go get the grain, and/or send Ukraine some goodies to make it too risky for Russia to occupy the western Black Sea.

I mostly expected them to try something intermediate, like intercepting with fast torpedo boats, subs or jets. Presumably they gamed that out and concluded it wouldn't work? Then again, this is Russia so maybe they just fucked up.