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Nevermind I already figured it out: In the script that sets the velocity in the animator, you need to check if you are rewining. If so, don't set any values and the animations will be played in reverse: 

    if (GetComponent<timeline>().timeScale <= 0f) {

I would also be interested in hearing a possible way to rewert animations from a blend tree. Much Thanks for releasing the Chronos for free in the asset store btw!