I am working on a joined report in which I want to summarize by a field that is available to one report block only. I have two blocks, i.e. two reports that I join. Help appreciated.
If you have a field that is only available to one report block, you can create a regular custom summary formula field for that block only.
If you are looking to create a cross-block custom summary formula field, to summarise across both blocks, you can't - the field needs to be common to both report blocks.
More information is available in the Salesforce help at: