France blocks Italian trains carrying migrants

A deal that more and more turned into blackmailing.

A deal that said you can keep the people we have no actual obligation to take unless you increase the fees you pay to keep them in our prisons
No blackmailing there at all, but I suppose the blackmailing was really hidden between the lines in a secret document with invisible ink that only a person of Skybirds intelligence can see.

This is just the beginning ... there will be a similar situation, which is probably already happening at this moment

Yes as surely the Shengen agreement doesn't cover this eventuality.