Hello romfeo,
I just set up a test project with the same landscape material blending approach you are using. The first thing you will want to make sure is that your project is set up for mobile platforms. To check this within your own project, go into your ‘Project Settings’ and look under the ‘Target Hardware’ section and be sure it is set to ‘Mobile/Tablet’ and ‘Scalable 3D or 2D’
Target Hardware
The next place I suggest looking is your landscape material properties. There are options you will want to check/uncheck under ‘Usage’ and ‘Mobile’ sections.
Landscape Material Properties
After checking all of these settings, hopefully are able to see your blending like in the picture below.
Mobile Landscape Preview
If you are still having issues after following the provided suggestions, please let me know and we can continue to troubleshoot your issue.
Cheers,