The Blackfire library is already available in PHP, but not loaded by default.
To load blackfire, simply add a conf/blackfire file with content from the Blackfire website.
Copy/paste the contents from the env vars including their names in the configuration file conf/blackfire (make sure you are logged in).
./bin/dev/down and ./bin/dev/up to load Blackfire.
Make sure to list all 4 env variables;
BLACKFIRE_CLIENT_ID=
BLACKFIRE_CLIENT_TOKEN=
BLACKFIRE_SERVER_ID=
BLACKFIRE_SERVER_TOKEN=