You can use toString() of Blob class.
Example:
String myString = 'StringToBlob';
Blob myBlob = Blob.valueof(myString);
System.assertEquals('StringToBlob', myBlob.toString());
Other option is by using String.valueOf() of String class.
There is one answer of sfdcfox which you might want to check before doing anything with blob. It says:
String.valueOf is always safe to use on a Blob, but Blob.toString may result in an exception if the contents are not a Unicode String.