404 blog url's delivering 403 status
After looking at the source code of the blog module, it appears that the path /blog/&lt;somestring&gt; is sort of reserved for the blog module itself.

Problem:

The pattern based aliases created when creating a new blog node seem to take precedence over those internal paths so if your URL alias pattern is blog/[node:title], you are shown the expected blog node as long as it exists.

Once the blog node is deleted and the alias is deleted with it, /blog/&lt;somestring&gt; means "do whatever path &lt;somestring&gt; does in the blog module" which results in a 403 Forbidden error rather than the expected 404 Not Found error.

Solution:

The only solution I found is to use a different URL alias pattern such as blogs/[node:title] which avoids the problem.

Tip:

Use the Redirect module to let the Pathauto module create redirects when changing URL aliases. It also helps to keep track of those redirects so you can see which redirect URLs still receive traffic and which could be deleted.