How do I get a form field value out of a config entity data blob?
The entity is already available with $form_state-&gt;getFormObject()-&gt;getEntity(). That basically answers your question.

Note that stuffing arbitrary keys into a config entity is not supported, you should use the third party settings API instead. See http://larowlan.github.io/meet-third-party-settings-interface/#/12 for a simple example (but make sure you also check the slides before about config schema). The initial part is not necessary anymore as all config entities support third party settings out of the box now.