Flock supports most standard blogging API, but we recommend metaWeblog because it is the most widely used.
http://www.xmlrpc.com/metaWeblogApi
We also strongly recommend to have the calls done over https, otherwise your users' login and password will be sent in clear.
The methods used by Flock are:
Add a link to a RSD on the front page of each blog. This will tell Flock what API your service supports and make automatic configuration possible. Users will just have to input the URL of their blog, their login and their password.
http://en.wikipedia.org/wiki/Really_Simple_Discovery
For a good example, check any blog hosted on wordpress.com.