The API allows you to leverage the RE7 Web Service in order to pull data back from RE, however there are no direct API calls that would allow you access to RE data beyond some very simple user information.  The process to build a custom directory would be:

 1.  Build a custom ASCX control that can plug into BBNC as a custom part. 

2.  Build a custom .Net Assembly that leverages the RE API (note,  RE is not a typo) and returns the data from RE that you would like.  This custom RE assembly can be placed in the custom folder of the RE7 Web Service

 3.  Your custom control, built in step 1 can then make calls through the API to the custom methods that you created in step 2.

This is a really brief overview of the steps required.  The labs site has some working samples that pull data from RE, through the RE7 Web Service.  See this link:

http://labs.blackbaud.com/NetCommunity/open

 and follow the link to 'Quick Start: Custom Part Samples', which will allow you to download the sample .NET solution that contains 6 samples, from a hello world web only plugin to a custom part that leverages the RE7 Web service.