I'm struggling for hours with this simple task.

I can select the area and change its fill successfully. But I want it transparent.

I tried changing the alpha to 0. I also tried changing the opacity under Blend Mode to 0.

These properties are saved correctly, but the area still doesn't seem to be transparent. When I open the file in GIMP, the area is white. When I use the SVG inside an HTML page, the area is white.

What am I doing wrong?

As you know how to change its fill, the best option is to remove the fill entirely by using the "X" at the far left of the palette at the bottom of the screen, or the "X" button at the left in the Fill & Stroke dialog.

If you still have problems, you'll need to provide some more information about your specific workflow, your version of Inkscape and, preferably, an example file.

Thanks, Xav.

The problem was actually that there was another path overlaying my path-of-interest.

The other path was the size of the entire canvas, and indeed included all the white spaces that I wanted to turn transparent (this was a bitmap trace).

How could I have seen that another area is covering my area? This isn't apparent at all from the object list. Any tip?

In recent Inkscape versions it's been possible to enable a checkerboard background, as commonly used in transparency-supporting graphics software, via the Document Properties dialog. It might not have helped in this specific case, but might have made it clearer that some parts were transparent whilst others weren't.

Are the two objects the same size and position? If they aren't it's clear from the bounding box markings what object is selected.

And as Xav said, checkerboard background helps.

The Objects dialog does identify imported raster images. They are indicated by "image123". (If you have a file where an imported image is not indicated in Objects dialog, it might be a bug.)

For myself, the checkerboard background interferes with ability to see details. So I just rely on my own memory (old as it is becoming), as to what is in the file.

One trick that I use a lot (especially in files that I'm not familiar with) is the Tab key. When the Selection tool is enabled, the Tab key selects one object at a time. So you can cycle through all the objects in the file. It even selects transparent objects. Then the status bar tells what kind of object it is. (Shift + Tab cycles backwards)