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.

"What if" is irrelevant because they won't but the "then what" is they would cry about that too and demand we stop doing that too and continue their impotent tantrum that the world continues to not let them do what they want to do, namely getting the Soviet Union back. Russia is impotent. Their threats don't get them what they want. Their military doesn't get them what they want. And most importantly, because I think I know where you're going with this, their nukes do not get them what they want. Their nukes only get them dead, which is something Russia only wants to happen to a large subset of their population, notably not including their leadership, and nukes are very dangerous to them and not something they will ever consider because their lives depend on it. Their clandestine political activities almost got them what they want and worked in several other situations, and it makes sense that they will continue to exploit those as much as they are able to. Nukes are not actually on the table, no matter how much they shout about them.