Here are 2 short videos giving hand on demonstrations of how to upload a video to a Moodle Blog or discussion forum. The first video shows how to do it from a Laptop or Desktop computer whilst the second shows how it can be done from a mobile phone. The demo uses an Android phone but it should work very much the same from iOS or other devices.
If you have problems doing it this way you can also post a link to YouTube video. If you do this, the video must not be set to 'private'. Instead set it to 'unlisted' and it will then not be visible to anyone without the link.