"stole item after PayPal refund"?? It sounds like they're accusing you of 'stealing' the item after they received a refund from PayPal. o_O

If 'blackmail' means the buyer threatened you with negative feedback if they didn't get something they weren't entitled to (eg to keep the item and get a refund), then it would qualify as feedback extortion and the feedback should be removed (if the buyer sent the messages via eBay, and eBay agree it is FB extortion).

Failing all other options, you can apply for removal on the grounds of defamation. You will need to submit this (witnessed) statement: http://pages.ebay.com.au/help/community/defam-form.html

EDIT: more options for reporting buyers: http://pages.ebay.com.au/help/sell/report_problem.html