Blister copper is:

The solidified copper obtained after bessemerisation is impure and contains $\mathrm{Fe}, \mathrm{Ni}, \mathrm{Zn}, \mathrm{Au},$ etc., as impurity. It has blistered like appearance due to the evolution of ${\mathrm{SO}}_2$ and so it is called blister copper.

$2{\mathrm{Cu}}_2\mathrm{O}+{\mathrm{Cu}}_2\mathrm{S}\to 6\mathrm{Cu}+{\mathrm{SO}}_2$

In sand moulds where the molten metal is cooling, ${\mathrm{SO}}_2$ is released. The "Blister copper" metal, which is 98% pure, is a form of impure copper.