Why are Safari Extension toolbar icons blue?

Solution 1:

The color is decided by the developer and signals that the extension is active. For example, on the favorites menu (blank tab) all my extensions are inactive and grey:

While when on a website, my extensions are active and display the programmed color. It isn't always blue. For example, Grammarly is green.

Edit: incorporating Mathias Winkleman’s answer, some developers choose default color, which is your accent color. By default it is blue, but grey does not change the icons to grey.

Solution 2:

In addition to what @anonymous said, here's how you can (lightly) influence the color: the default, blue in your case, follows the setting for Accent colour in Preferences ▶ General.

One of the options for that setting is grey. Unfortunately, that happens to be the exception, with icons reverting to blue when it is selected.

If, however, you never actually need to click on the icons, you can right-click on the toolbar, choose customize, and remove them. The extensions will continue to work as before.