First an opinion
Drupal's strong point are flexibility and modularity.
Drupal's weaknesses are heaviness and steep learning curve.
The design you are proposing does not take advantage... of the advantages and leaves you with the drawbacks, so IMO - although you can definitively do that - it's not the greatest of ideas, at least judging from the few elements provided in the question.
I would rather look to CakePHP instead, which is still PHP but does natively what you want to do (i.e. having the framework handling the "logic" but letting you writing your own presentation layer). It is also incredibly lighter and faster than Drupal.
That said...
You can do it with Drupal
The easiest way is to use the API's provided by the BlogAPI core module. That is an out-of-the box functionality of Drupal. For anything more complex you could still use the webservices with defining your own functions. If you are not new to drupal (thus you don't have to spend lots of time in learning how views works), a powerful way could be to expose views through the xmlrpc server.
Reading from the DB directly is also a possiblity, but it gets very complex, very quickly (I would say it is at least an order of magnitude more time-consuming and bug-prone than using a webservice).
HTH!