Recently I notice a blue dot above “aA” in the Safari address bar, when loading a page. It disappears after a few seconds.
What does this blue dot indicate?
Just for kicks I asked ChatGTP. Below is the answer, which I’m not prepared to vouch for. 
In Safari browser, the blue dot above the aA icon in the address bar indicates that the website you’re currently visiting has an active Reader mode available …
Why would the dot disappear so quickly if that answer is/was right?
I have no clue. 
I’ve never noticed it. Usually, I see this on the Mac:
PS: I found this but it is not helpful.
It might be helpful, if you share a screenshot of this behavior.
I thought it had something to do with an installed extension wanting something from you 
I believe it means there’s an extension that can interact with the current page.
It is not showing up, if I call relay.fm from my Xs.
So what might be the differences?
As @JensV and @onepointzero have suggested: it might be a Safari extension you have installed showing you its availability for the website. I think this might be what is going on here. Tap on aA when the dot is shown and have a look at your extensions in the list. Blue ones are active, grey ones are not active.
Be it as it may - this is the lovely UI these days: a community of “power users” rack their brains over the question what iOS is trying to tell us. This is how a good UI is working. Not. 
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If not the new extension, quick googling reveals this might be caused by voice control being on and the app expecting voice input. Go to Settings > Accessbility > turn off Voice control.
That is a different one, I think:
https://discussions.apple.com/thread/250730912
(Voice control has a blue circular microphone symbol in the top right area of the display as it can be seen in the pictures in the linked topic.)
I think the dot is displayed by Consent-O-Matic when it’s performing actions on a page.
(I don’t see the dot when I disable this Safari extension)
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