ID:
Any valid ID (18-character force.com identifier). If you set ID to a 15-character value then apex converts its value to its 18-character representation.
Note: All invalid ID values are rejected with a runtime exception.
Blob:
A collection of binary data stored as a single object.
A blob is typically used to store images, audio or other multimedia objects and sometimes binary executable code is also stored as a blob. We can convert this datatype to string or from string using the toString() and valueOf() methods.
Ex: String s = ‘abc’;
Blob b = Blob.valueOf(s);
String s1 = b.toString();
