How do I undo the command $ eval “$(docker-machine env blog)”

What you are looking for is:

eval "$(docker-machine env -u)"

It will unset the DOCKER_* variables.

For the record, here’s the output of docker-machine env -u:

unset DOCKER_TLS_VERIFY
unset DOCKER_HOST
unset DOCKER_CERT_PATH
unset DOCKER_MACHINE_NAME