When I create a new site, the SitePages are always of "Wiki" content type.
Is this normal and correct? I would have expected them to be "Web Part Pages" by default?
Team Site and Blank Site use the same basics, the difference is that Team Site has the "Site Pages" created by default, and it's base content type is the wiki pages. It's not created by default in blank sites.
By default blank sites use the webparts pages (Also as the homepage for the site), until you do one of the following: 1- Create a new wiki page, which will create the Site Pages library. 2- Activate the "Wiki Page Home Page" Site Feature on the blank site, which will create site pages library and change the home page of the site to a wiki page instead of webpart page.
So you'll have by default the wiki page library which is intended for Wiki Pages, not webpart pages.
Are you by any chance creating a wiki site?
There is a "Basic Page" content type available. You get a Wiki Page each time only because its your page library's only or default content type.
You can try to create a Document Library and allow content type management. Then, add the "Basic Page" into your newly created document library and set it as the default content type. Then, when you create a new aspx page in SharePoint Designer, you will get a blank page
https://stackoverflow.com/questions/7870135/why-is-wiki-page-the-only-option-for-a-new-page
also:
In SharePoint 2010, however, the home page of most web sites is a wiki page, and it is stored in a new type of library called Site Pages. These sites also have a library called Site Assets, where files like images for the wiki pages are stored.