Conditional Formatting Not Working Online

I'm using Conditional Formatting to color the background of some of my fields. First, I created a new Column:

Background Color = if(not(isblank(Table[Column Name])),
                                    "#000000",
                                    "#FFC0C0")

Then I use Conditional Formatting to set the Background color of a particular field in my table using "Format by Field Value" and setting the field to the Background Color column I created above.

Great. That works perfectly. But when I publish to Power BI online, the color completely disappears. Using 'Edit Report in Power BI Online, I can see that nothing has changed - the field still has the same value for Conditional Formatting. So why doesn't it work?