Signs according to who? Ukrainian intelligence? Or do they call it the Abwehr?
“Signs according to who? Ukrainian intelligence?”
That’s why it’s an IF, and not — at least at this point — fact.
And your comparisons to the CMC in 1962 and Britain in WWI are wholly inappropriate, as they dealt ONLY with interdicting shipping into a specific port or ports of a specific country; and, moreover, both predated the Convention of the Law of the Sea, which went into effect in 1982.
The premise in the article was that Russia could be planning a COMPLETE blockade of the Black Sea. To make your comparisons comparable, the entire Atlantic Ocean would have had to have been blockaded in WWI and in 1962; and, of course, that never happened.