Django 2 namespace and app_name
try removing app_name='blog'
In your case you should be using:
'blog:post_list'
and
'blog:post_detail'
You can also remove the namespace='blog'
in your first url like so:
urlpatterns = [
path('blog/', include('blog.urls')),
]
and then in your templates you can reference the urls without the 'blog:.....':
'post_list'
'post_detail'