I tell people: In some cultures the word bless means "fuck".

It depends on the individual's understanding and intended use of the phrase.

I recognize that I'm very fortunate in some ways ... this is just recognizing and appreciating what I have that's positive. But ... it could conceivably involve some arrogance or bragging if I develop the notion that this is somehow a validation of superior personal worth, or if I got off on others feeling inadequate around it.

The problem is you don't know me so you can't judge all that with perfect accuracy. If I kept bringing this up out of context at every opportunity or tried to diminish others by causing them to suffer in comparisons I set up and make, that might be a "tell". But ... only I know my actual motives, right?

I think also that some of the prattling on that people do is just that they want to tell themselves that they've built a zone of safety around themselves, that they're in some way entitled to their achievements. When in fact circumstances can cause anything to be taken from us, at any time, for any reason or for no reason. When you have little or nothing, there's anxiety around your lack; when you lack for little or nothing, there's anxiety around retaining it.

I see it as a cultural colloquialism that for nonbelievers, translates as "I'm fortunate."

Yes this exactly!!!!

On a related note, every time someone tells me to "have a blessed day" it makes my skin crawl.

Maybe, unless it's an obvious expression of gratitude.

It's like being a Star Belly Sneetch.