This isn't an exact feature by feature comparison between products.  If you want that you might want to contact K2 directly as I've seen various documents around that might help out.

From a release standpoint, K2.net 2003 is the earlier version of K2 blackpearl without SmartObjects and without many integration capabilities besides writing custom code to call web services or .Net assemblies.  As the name would imply K2.net 2003 was released roughly in 2003 on the .Net 1 framework but updated to run on the .Net 2.0 framework. K2 blackpearl was released in August 2007 and K2 blackpoint roughly about a year later.  K2 blackpearl and K2 blackpoint have been updated continously since then.

In a nutshell K2 blackpoint is a subset of the K2 blackpearl feature set and is priced accordingly. Some of the major differences is K2 blackpoint doesn't contain all the Service Objects K2 blackpearl has which limits its integration capabiltiies. K2 blackpoint also doesn't allow you to get to the code level or allow customers a license to utilize the APIs which limits its extensibility and flexibility when compared with K2 blackpearl.