So my mom is 80 and has dementia.

If there's something emotional attached to a memory, there's a greater chance that it will be remembered.

So, the blackout could have been a bit scary or uncertain for her.

Yep. That's how it goes with my mother, for sure.

Definitely. Also, with the illness my mother started regularly lying, knowingly that she is lying. Sometimes it becomes obvious that she "doesn't remember" when she's being obstinate/frustrated, so there's an element of that too.

Yes, NONE of it can be "understood" by the rational mind.
Get all the education you can.
Start with Teepa Snow videos.
The internet is just packed full of stuff for you.
Oliver Sacks said of dementia that "they have an entire world; it just isn't YOUR world" and that's exactly it. You are looking at an alien at this point. And your goal is to be as understanding as you are able of "her world", which makes no sense whatsoever in your own.

She probably remembered the blackout because that was something more out of the ordinary compared to eating a very recent meal. People with dementia are more likely to remember events that had some sort of emotional charge tied to it.

That does not mean they remember those events in the same way we do. The emotional part might stick in their brain, like having to stop driving, but they will forget or not be able to comprehend why they can no longer drive, so they will continually bring it up.

There is no rhyme or reason to this disease . Yes many of us have seen and heard inexplicable things with dementia just as you describe . It doesn’t make any sense for sure .

My mother could not remember to change her clothes , wash up or eat , but she could still come up with manipulative plan such as calling 911 and telling the police I was stealing her money because she was angry that I put her in assisted living . She could remember she did that and admitted to me that she did that because she was angry with me for placing her in assisted living .

Before I placed Mom in AL when I was trying to get her to eat, wash up and change her clothes , she got angry and was hitting me and pushing me . I told her I was going to call the police . Without missing a beat she said “ Go ahead , call , I’ll tell them you hit me . Who are they going to believe you or the old lady ?”

Another time ( again before she was in assisted living ) she tried to push me , when she did she lost her balance and fell . She told my siblings that I knocked her down . She remembered she lied .

She could do all this but insisted she washed , changed her clothes and ate the meals I made . I was finding the meals uneaten in the trash . She stunk, was in the same pajamas . She was living on cookies . She wouldn’t do anything I asked , nor would she let hired help in . So she had to go to assisted living .

I can only guess that you've not done much research on dementia, as with all of the dementias the short term memory is the first to go and the long term memory hangs on for quite some time.
I would recommend that you start educating yourself about this horrible disease, as your mom will only continue to get worse and you want to be prepared best you can.
You can start with reading the book The 36 Hour Day, and then look up Teepa Snow(a dementia expert)on YouTube, as she has many great videos there, plus she's written many great books about dementia as well.
You are in for quite the journey with your mom, so hang on tight and get your patience ready as it will be tested over and over and over.