Sharepoint - specify the default color for calendar event

The calendar colors are based on your sites style/theme. So my guess is that you are using a non default theme (or perhaps you created your own) on the sites that are showing black, and the default one on the one that shows blue. If you click the gear in the top corner, then click change the look, you'll see the color palette for each theme. Typically the color used as Accent 1 will be your default new event color.

In context of your comment on the above answer It seems that you want to change the color of calendar event without changing the theme. I am having an approach for you. As I achieved it near past when I was having requirement to change the color of Calendar based on the event category.

What I did was,

• I edited the page on which calendar view was placed.
• Kept a Content Editor WebPart

• Added following code into it.

  $(".ms-acal-item").css("background-color", "orange"); // You can use any color you want
  

• Save the page.

By doing this my calendar looked like

I know this is not the ultimate solution. But I guess it may help you to achieve what you want to achieve.

Let me know whether you need some more assist on this.