Black bars appear when a snippet isn't the same shape as the mash, on iOS and Android alike. Phone cameras take photos at 4:3 and video at 16:9, so a photo can't fill a video-shaped frame, and a landscape video in a portrait mash gets bars above and below.
The mash renders in one orientation, set with the Orientation button in the mash screen; each snippet's framing is chosen when you add it. To keep black bars to a minimum:
Pick one orientation for the project and choose snippets to match it.
When you add a snippet, choose the orientation that matches the project in the Select your Snippet screen.
Photos darker than the videos around them on iOS? Turn on HDR in the mash screen.
Not a fault: exporting from the fullscreen player on a mash card always gives a tall video with white borders, whatever orientation and background you chose. To export with your settings, open the mash with Edit Mash and use the share arrow in the mash screen.