The distinction between <block>s and <datasource>s is not exactly intuitive. The
table name is defined in <datasource>, the field list is defined within the
<block> (but explicitfields can also be defined in <datasource>, while sortorder
is defined in the <datasource> only. Filter conditions can be defined in
<datasource> in GFD and for the <block> at runtime.

It might make things easier if table, sortorder and conditions can also be
defined within the <block>, which might even make the explicit <datasource>
obsolete for most cases.