I have a weird question. Some numbers of pips can have the black pip. However, the 4 captain pips have never been depicted with this.
In some ways, this makes sense; a "lower" captain wouldn't make sense, and we've seen that the highest first officers hold commander pips. It's most likely that have 4 pips with one black is totally invalid.
However, I wonder if there's ever a circumstance where the black pip would be there. For instance, let's say someone gets field promoted to acting captain, but Starfleet either takes their time making it official or it's going to take a while (a few weeks) for the ship to get back to starbase to pick up a new captain (meaning the acting captain will be a bit long-term)? Could it be used then?
I imagine most of this is speculation, but I'm wondering if there's any example in canon of a long-term acting captain that could disprove the use of this pip configuration.
Sub-caps are the worst.