I doubt this is a OOjs issue, rather i suspect it has to do with htmlforms not translating the disabled attribute correctly or something. See also T153636: OOUI form fields do not set the dir attribute of textfields for a similar situation regarding the dir attribute.

Change 328370 had a related patch set uploaded (by Huji):
Disabled checked checkboxes in OOjs-UI should look grey

https://gerrit.wikimedia.org/r/328370

@TheDJ: I manually tweaked the HTML element and made the checkbox disabled (i.e. didn't let HTMLForm get in the way). It was still blue; I explored the CSS and turns out there was no rule for checked disabled checkboxes, so I added one.

Screenshot?

Before:

After:

This could be programmatically a special case not covered by OOjs UI yet (but probably not recommended or desired either). If a CheckboxWidget is set to disabled, the corresponding label should be disabled as well. Not just the checkbox. That's the current implementation (see “CheckboxInputWidget (disabled)‎”.

Change 328370 abandoned by Huji:
Disabled checked checkboxes in OOjs-UI should look grey

Reason:
Wrong repo. Will submit to the correct repo.

https://gerrit.wikimedia.org/r/328370

In T153752#2890237, @Volker_E wrote:

This could be programmatically a special case not covered by OOjs UI yet (but probably not recommended or desired either). If a CheckboxWidget is set to disabled, the corresponding label should be disabled as well. Not just the checkbox. That's the current implementation (see “CheckboxInputWidget (disabled)‎”.

Perhaps I should ask before I submit a new patch. My understanding is that if a checkbox is checked, it's parent element will be assigned the classes oo-ui-widget-enabled. So enabled here has to do with checked or not checked. Is that correct?

Change 328393 had a related patch set uploaded (by Huji):
Disabled checked checkboxes in OOjs-UI should look grey

https://gerrit.wikimedia.org/r/328393

Change 328393 abandoned by Huji:
Disabled checked checkboxes in OOjs-UI should look grey

Reason:
You are correct; I was initiating the element incorrectly. Abandoning.

https://gerrit.wikimedia.org/r/328393