Yep, from their blog.

"2:04am PST – Criminals are attacking feedly with a distributed denial of service attack (DDoS). The attacker is trying to extort us money to make it stop. We refused to give in and are working with our network providers to mitigate the attack as best as we can."

http://blog.feedly.com/2014/06/11/denial-of-service-attack/?...

A quick google shows 'blackmail' definition can either be restrictive to secrets as you say or can encompass 'extortion' depending on the definition you use.

It's not blackmail, just plain old extortion.

They are probably saying "Give me money or I am going to continue DDoSing your site."