How do I output the field B or a fixed value if the field A is blank?

It is not possible to set conditional branching logic for merge fields, such as "If the field A is blank, output the field B or a fixed value in the merge field". Create a formula field in Salesforce and set the formula field as a merge field.

As an exception, checkbox field values can be replaced True/False with other values.

e.g. {!Opportunity.IsDelivery__c, Yes;No}

Instruction manual

Replace True/False in Check box Fields with Other Values