I don't see anything directly in the API at first glance (which doesn't mean it's not available - I just don't know for certain if it exists).
It appears the stored procedures in place are written for the logged-in individual to see their own subscription status (and subscribe or unsubscribe), and then one that returns a bulk batch of email addresses which I assume is implemented by the job scheduler when it sends out the notifications of new posts for the blog.
If you need to do some quick adhoc reporting and have access to your database:
It looks like the subscription information is stored here: te_Blog_BlogSubscriptions
And you can get a list of emails with this SPROC: te_Blog_BlogSubscription_GetEmails
If you are using Analytics 4.0 you could create a velocity extension that would then call this sproc and return the email address count for the blog in question. Then you could create a widget that displays this data and add it to any page. For a richer set of results, you could query the table above and return a set of results with blog names, counts, etc.
Not supported and not via the API, but looks like that is the location of the data.