A BLOB field is simply a field that stores data in binary.
A Binary Large OBject (BLOB) is a collection of binary data stored as
a single entity in a database management system. Blobs are typically
images, audio or other multimedia objects, though sometimes binary
executable code is stored as a blob. Database support for blobs is not
You can view the attachments in ArcMap in two different ways.
One way is using the Identify tool on the feature, and then selecting the file from the Attachment drop down:
The other way is by selecting the feature, and then selecting the file from the Attachment drop down of the Attributes window:
But as far as joining your point feature class to an excel file inside the BLOB field, that's not going to work. As mentioned before, data inside the BLOB field is stored as binary.
You could probably write a Python script that exports the excel file to a temporary location and does the join, but that's a bit more work. Here are a couple sources for reading/writing to BLOB fields with Python:
Batch export attachments from a feature class
Working with BLOB data