There is no blob field type in Salesforce. A couple of thoughts here. Either you can store it as an attachment related to the record, or store it as a very large string, hope the blob doesn't exceed the size of the string, and handle any encoding / decoding through triggers, etc.
@satya, What is it taht you want to achieve? maybe more info can help us https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_primitives.htm https://developer.salesforce.com/forums/?id=906F0000000Ax5sIAC
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_primitives.htm
https://developer.salesforce.com/forums/?id=906F0000000Ax5sIAC