What are the reasons that I can't edit a page in SharePoint Designer?
I am using a publishing page with a blank web part page layout.
you cannot edit a page in SPD which has a page layout attached. you will need to edit the page layout, or copy the markup from the page layout to the page itself using "detach" option.
The reason is that the page has an attached page layout which it depends upon. Either you edit the page layout (and effect all other pages relying on this page layout) OR you detach the page from the page layout.
If your going to the second option, for valid reasons, right-click the page you want to edit and select the
Detach from Page Layout option:
You'll get an informative warning message by the console which explains what will happen.
Select Yes, and the contents of the Page Layout will be added to your page.
Now you can right-click the page again, and select Edit File in Advanced Mode.
NOTE Always make a copy of the file you're about to edit so you can go back to a working page, if you (or SharePoint Destroyer) mess things up.