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'