What is Blob in Salesforce? Where it is Used?

Hi Suraj

Blob is a collection of binary data stored as a single object. You can convert this data type to String or from String using the toString and valueOf methods, respectively. Blobs can be accepted as Web service arguments, stored in a document (the body of a document is a Blob), or sent as attachments.

Blob Methods
The following are methods for Blob.
size()
Returns the number of characters in the Blob.
toPdf(stringToConvert)
Creates a binary object out of the given string, encoding it as a PDF file.
toString()
Casts the Blob into a String.
valueOf(stringToBlob)
Casts the specified String to a Blob.

Hope it may help: