@Navalland I am not super familiar with map making. So I cannot reproduce what you just have reported. I am not sure what “View Map Blend” has to do with basetiles and relieftiles.
Please take a look for example at the wwII-v3 map.
The basetile image defines the territories, seazones and borders only. Those are relevant for the polygon grabber.
The relieftile image only adds another partly transparent layer that brings for example the streetmap-like view and the different blue scales of seazones.
In the map.properties file you define the pixel size of your map. This pixel size must be exactly the same as the size of your images.
In the image editor software you now would work on images of this size.
You would now create a base layer (basetile) and draw the outlines of the map. After that you would add another layer (with tramsparency ) on top and add for example the cartography and other stuff you might want. This additional layer is the relieftile image.
You would save both layers as separate png-files. The image breaker is used to break the basetile-image into the baseTiles directory of your map. Then you break the relieftile-image into the reliefTiles directory.
Maybe you know all of this - I cannot tell from what you wrote. But the above is a very short “how to” .
Also the “View Map Blend” option seems to be independent of these image files.
However my knowledge of this process ends at this point, so in case you have further, more detailled questions I recommend to look and ask here:
https://forums.triplea-game.org/category/28/map-making
https://forums.triplea-game.org/topic/918/map-maker-resources