How to change photo album background color from white to black

In Photos 3.0 you can change the color of the background of a page like described here:

Arrange photos on a book page in Photos on Mac - Apple Support

Change the layout of a book page

1. Click a book under Projects in the sidebar.
2. Double-click the page you want to change.
3. Click the Options button below the page you want to change.
4. Scroll through the available page layouts, then click the page layout you want.
5. The book theme you use determines the available page layouts.
6. To change the background color, click a color in the color bar.
7. 

It will depend on the theme of the book you picked, if black is available as a background.

Once your book is ready, you can save it as a PDF file. Then upload the PDF to a third-party prinin service. For example, Presto Photos can print Apple Books from the PDF you saved, see:

Photo Book Printing, Color Book Printing, and Calendar Printing - PrestoPhoto

To create the PDF file, see:

:
Preview a print project in Photos or iPhoto to avoid issues - Apple Support

Hi Julait! There's no way to do what you want, as far as I know. But, if you have updated to macOS Mojave, then you can turn on the new Dark Mode feature, and then Photos 4.0 will display everything with a black background, which I personally think looks wonderful! I know this isn't exactly what you want, but it´s all I can think of. Hope this helps you! Regards, Ian.

I've just finished an album

Do you mean a standard album of photos or a slideshow or a book?

In book projects you can change the background, but Apple does no longer print books.

Which version of Photos are you using?

Use an online service where you can choose any background you want, and change it part wat. Vistaprint, Photobox etc