ok... here's a question...

is your entire level composed of blocks that share a single texture?

That's the only way I can think of that could duplicate the things you are describing with the different texture flags.

If you want a block to be invisible, but still obstruct movement, select a placeholder texture (one that you would not use for any other item in your level) and set the texture flag to 'none' under that texture's settings. Now, any surface you apply that texture to will not be rendered but will still block movement.