XML is the ability to add structure to databases, much in the same way HTML brought structure to documents. The idea was to allow anyone to design a schema to organize your data in some meaningful fashion, to enable searches, sorting of results and other useful activities. All requests and responses are coded using XML documents.
Blocks is a more complex architecture for managing metadata.
It consists of programs to examine and transform the data and then apply and store these transformations as structured objects or blocks. Up until now, databases and Web pages were an unhappy marriage. There were several ways you could use Web forms to query and display your data, but for the most part things were messy, complex and required some heavy custom programming. If you are doing this now, chances are you are using Perl scripts or JavaScript to perform queries and format the results for your Web pages. Computer Associates' Opal was one solution that implements Web access to mainly mainframe-based data.