Create a new layer on the Master page, and move it above all other layers. Put your page numbers (and anything else from the Master page that needs to be on top) on that layer.
You should work generally with layers. In my opinion it is a must in InDesign.
Build up your documents with at least following layers, bottom up: Background, Images, Graphics, Text, Page_Number.
This will help to get things right in production.
If I show you this, you're going to say: Normal! the text block is in the foreground.
But if I say: No, it is in the background, and therefore there is no need to play with layers, you will call me a cheater!
The text block is actually in the background. As much as a page number, treated without further precaution! ...
When you change the image blending mode to overlay you will get this result, but you can encounter other problems. E.g. spot colors will change to process colors.
Hi Peter, Hi Willi,
Indeed, there is an effect on the image block ["Incrustation", in French].
This works fine with primary colors. Here, we talk about page numbers ... Maybe black!
What I want to say is there're sometimes other options!