Since there is no response to my question, guess i have to stick to my own findings
https://groups.google.com/forum/?fromgroups#!searchin/tumblr-api/Private$20Blogs/tumblr-api/qySxi23DdXI/w0NB0qxxrGIJ
this is a link i found that states that we should perform a complete oauth for accessing private blogs. Our idea was to keep the source of posts secret. So we managed to get the oauth token and secret of the admin of the blog , and then we hardcoded these two strings so that we have all the 4 keys and we can access the posts from the private blog and without the user knowing about it (without the actual oauth). we had contacted tumblr and made sure that the auth token and secret donot change.
About accessing the blog if password is known, didnt find anyway to do so. Dont know whether it exists.