Turkey did not start the war in Syria, but they do share a border with them, and have a responsibility to secure their border from a failed state. They took in more refugees than any other country in the world, which has a massive impact on their economy and security. Requesting economic aid from the EU to deal with this is not blackmail. The EU benefits massively from keeping Syrians in Turkey, because Syrians primarily use it as a pass through on their way to Europe.

"and have a responsibility to secure their border from a failed state."

https://en.wikipedia.org/wiki/Turkish_involvement_in_the_Syr...

So you think supporting islamist militias is helping stabilize said neighbor state?