You’ll see it a lot on the internet; cat lovers discussing how their cat does the classic ‘slow blink’ at them and happy to say that it is a sign that their cat loves them. In general people consider the cat slow blink to be a sign of affection from their cat.
It is, in a way but I think its origin indicates that its meaning can be slightly different.
The blink stops a cat staring. And a prolonged stare is intimidating to another cat particularly a subordinate one. The subordinate cat might withdraw.
Therefore in order to maintain social relationships and the benefits that come from that, non-aggressive cats when staring at other cats and their human companion will blink which is a signal that they are not hostile. The blink breaks up the stare.
To be non-hostile is not quite the same as being affectionate. However, it probably has evolved in human households to mean that.