and there is this copy styled text feature in the search menu,
which could be used within the macro if more than one block of text is highlighted

Actually, if Multi-Editing is enabled, the original macro handles “more than one block” just fine, no need to dabble in “styles” or “Copy Styled Text”.

Most useful for you would be an “Invert Selection” command, that would take your current selection and select everything else. Then a tap of the Delete key and you are done.

Alas, there is no such command in Notepad++.

Plus, it is complicated, because it would require “multi-select” to be enabled, because in most cases, you would need such a selection inversion command to select a block before your selection and a block after your selection.

But all is not lost…

A simple thing that seems to work is to record a macro for the following actions, in order:

• Copy (Ctrl+c)
• Select All (Ctrl+a)
• Paste (Ctrl+v)

Then you give it a name and assign a keycombo to it.
When you want to actually use it, you select the text you want to keep, then run the macro via the assigned keycombo.

Of course, the slight downside may be that it destroys your clipboard contents, but perhaps this is a small deficiency.