 In this short video I'm going to show you how to mosaic two tiles that have these black borders around them. Let's first look what these black borders are. If I use the Identify tool, I find that they have a value of 0. You can also check that for the other tile. I see that the border has a value of 0, while the values inside the tile are larger than 0. This method also only works if the border is 0 and there are no zeros in the rest of the layer. So then I go to Raster miscellaneous, I can build a virtual raster or use merge, the procedure is quite similar, I select the tiles, and now it's important that you fill in that value of the border, in our case 0, and also assign it a specific no-data value, and by convention that's often minus 9999, an out-of-range value, and then I save it to an output file and I run, and there's the result where we have the tiles merged seamlessly, and you can do of course the same with the virtual raster.