You are simply running into bandwidth issues… Blynk is not a dedicated video viewing app, but an IoT App that supports a degree of video streaming capability, but also has its own full data steam constantly running.
Lower the bitrate of the source, or pick a lower rated subtype in your public URL, and turn off audio feed (if any) from the camera… it will probably never be as good as running in your private network, but at least better than black.