Russia announced it will consider all ships sailing in the Black Sea to Ukrainian ports will as carriers of military cargo from July 20th onwards.
Will Russia dare to attack non-Ukrainian civilian ships? I think it will.
Will the world swallow what is essentially an act of piracy? Yeah, it will. I am 99.9% sure of that.
Anyone flying over or sailing nearby a warzone should take heed that objects on radar may look like wartime targets; it's more unusual to keep commercial shipping open between two belligerents and their trading partners than it is to just keep this blockade. If this was like China trying to claim the SCS and establish illegal control over open waters during peacetime there might be more point in complaining, but as it is the illegality of Russian actions is moot when this is all in the context of an already illegal war. Despite the Russian government maintaining that this isn't a war, this is fairly standard stuff for a war. It may not be in Russia's strategic interest as like you say, shooting a 3rd party nation's ship is politically disastrous, but look at the Falklands war - another one of those wars that never was openly declared, where the UK warned any ship within a certain radius of the Falklands would potentially be considered a combatant vessel. Noticeabley, even Ukraine is now suggesting they will retaliate with the same measures:
The defence ministry in Kyiv issued a statement saying that from midnight on Thursday, all shipping plying Russian-held ports “may be considered by Ukraine as such carrying military cargo with all the associated risks”. The announcement mirrored a threat from Moscow on Wednesday against all ships using Ukrainian ports.
“The fate of the cruiser Moskva proves that the defence forces of Ukraine have the necessary means to repel Russian aggression at sea,” the ministry said, in a reference to the sinking of the flagship of Russia’s Black Sea Fleet last year.
“Ukrainian-made maritime drones are capable of destroying stationary and moving targets anywhere in the Black Sea,” a Ukrainian interior ministry adviser, Anton Gerashchenko, said on Twitter.
While we haven't seen a war involving commerce raiding since WWII, we have seen three blockades in modern times. UK-Argentina, Falklands war and Iran-Iraq Gulf War, as well as the Saudi-led blockade of Qatar in 2017. In the former two both wars never saw an official declaration of any blockade, but did see this declaration of an "exclusion zone" where civilian ships were warned they could be mistaken for military ships, but it is in practice the same as a blockade. In terms of precedence, the Russians and Ukrainians are in the clear here