Currently it's not possible to update a datasource field value as blank.
Ideally it should be possible to "reset" a field value where required.
i.e. When a new record is created that has for example a date field, this field is initially blank. If you then update this field with a date value but then wish to reset this value, this isn't currently possible.
This is also true for numeric fields, where it should be possible to write back a blank/null value.
Although it is possible to set a text data field to "", which is simple to test for, this isn't true a blank/null value.
This feature would most benefit numeric and date fields.