Blister copper is a form of a impure copper which is obtained in self reduction process during bessemerization.

Reactions take place in blast furnace-

$2 Cu _2 O + Cu _2 S \rightarrow 6 Cu + SO _2$

The molten metal is cooled in sand moulds $SO _2$ escapes. The impure copper metal is known as "Blister copper" and is about $98\, \%$ pure.