John G wrote:
I have an old blacksmith's anvil, which through lack of use in a damp
atmosphere has become rusty

What would be a good way of restoring it? Sandblasting?

If you own a belt sander, then that can be an easy way.
Another is an angle grinder, with a metal cutting disk applied lightly
on the side of the wheel, in a smoothing action.
Better to use a sanding disk on the angle grinder though.


Alternatively, simply using for the original purpose will soon remove the
rust.
I'd suggest you forge ahead with this approach.