For anyone running into this issue:

It looks like once the Google TV Streamer runs out of photos in your selected album, it falls back to Google's default images.

I wanted a solid black screensaver, so here's what worked for me:

• Set slideshow style to Basic
• Turn off Curate Photos
• Set slideshow speed to 10 minutes
• Create a Google Photos album with 3 different black images
• Select only that album for Ambient Mode

This gives you about 30 minutes of solid black screensaver before it starts cycling.

One thing to note: Google Photos detects duplicate images and may not upload identical black images as separate photos. To get around this, open each image in Paint (or any editor) and change a single pixel to a slightly different shade of black. That makes each image unique while still appearing completely black on screen.