weird. I wish I had a better answer for you. I had not noticed the little gear for the Export for Screens save prefs, but now that you pointed it out, the svg settings are the same set as the ones displayed in Export.
When I try to use Export or Export for screens, all the svgs display tiny in discourse. Using Internal CSS with Export gave me the black-box issue you described (and still tiny).
Using Save As or Save Copy gives more options.
These are the Save As/Save Copy settings that I used which gave me an svg that displayed correctly in discourse, at the correct size.
Are you willing to try saving it again using these settings, and let me know your result? I want to nail this down once and for all. I hate that I passed along advice that didn’t work.