I Is there such a thing as an active diamagnet?

[cuallito]

Does something like an active diamagnet exist for when you need to block external magnetic fields, but a superconductor wouldn't be practical?

 

[vanhees71]

What do you mean by "active" diamagnet?

 

[Steve4Physics]

Does something like an active diamagnet exist for when you need to block external magnetic fields, but a superconductor wouldn't be practical?

If by ‘active’ you mean something that requires a power source (e.g. requires a current), then I’’ve never heard of one. For simple magnetic shielding, an alloy called ‘mu-metal’ is often used. E.g. see https://en.wikipedia.org/wiki/Mu-metal.