You can blend multiple types of sources. Excel, SQL database, text file, ...
This makes it very useful to add local / specific data to a centralized source. For example, sales data may be in the main SQL store, but sales rep targets can be in a local Excel file.
Im having trouble with that, im sure there is a common field for both my data sources.
But when combining the fields into one table, I always find one data field to be null.
Maybe you can post a subset of your data in a Tableau packaged workbook?
I assume you've been through the basic overview of blends here:
You can check that there is a linking field by adding a dimension from one data source---the primary data source---and then clicking on the second data source. There should be an orange chain link showing which fields are linked. By default, fields with the same name that are in the view. If you don't see any links, most likely the the dimension names don't match and you can either
1. edit the alias in one of the data sources,
2. create a calculated field with the same name, or
3. select Data > Edit Relationships from the top menu bar and use a custom "join"