This is why maritime security in the Black Sea – or rather the lack thereof – has global repercussions.
The maritime blockade is:
-undermining Ukraine’s economic viability and, therefore, its sovereignty and independence
-increasing global famine and
-increasing global costs of living, which means increasing the risk of demonstrations, riots, extremism and fall of government
-if unchallenged, undermining universal Freedom of Navigation worldwide.
African nations and China are going to pay attention to those who help them get their grain shipments. Russia's attempt at backing out of the grain deal has put a dozen country's interests opposed to theirs.
Russia is running out of functional allies and gaining more enemies. While not directly benefiting Ukraine it is bad for Moscow, which is good enough.
The maritime blockade
As a technical matter, Russia has not proclaimed a blockade, but rather a Maritime Exclusion Zone (MEZ).
https://lieber.westpoint.edu/maritime-exclusion-zones-armed-conflicts/
Enemy Merchant VesselsBelligerents may capture enemy merchant ships anywhere outside neutral waters for adjudication as prize. Enemy merchant ships operating within or outside the MEZ may not, however, be attacked or destroyed unless the vessel:
(1) persistently refuses to heave to after being ordered to do so;
(2) actively resists visit and search or capture;
(3) sails under convoy of enemy warships;
(4) is armed with weapons systems beyond that required for self-defense against criminal threats;
(5) is incorporated into, or assists in any way, the enemy’s military intelligence system;
(6) acts in the any capacity as an enemy naval or military auxiliary; or
(7) is integrated into the enemy’s war-fighting/war-supporting/war-sustaining effort (NWP, § 8.6.2.2).
In destroying enemy merchant vessels, belligerents must first place passengers, crew, and the ship’s papers in a place of safety, unless an enemy merchant ship persistently refuses to stop when ordered to do so or actively resists visit and search or capture. This requirement does not apply, however, if under the circumstances at the time of the attack, the warship would be subject to imminent danger or would otherwise be precluded from accomplishing its mission (NWP, § 8.6.2.2).
Neutral Merchant Vessels
A neutral merchant ship that enters the MEZ is subject to the belligerent right of visit and search by Russian and Ukrainian warships to determine the enemy character of the ship or its cargo, but it may not be captured or attacked unless it engages in certain prohibited conduct.