This might sound like a really dumb question and it just might be, but I can't figure out for the life of me how to easily create a simple fillet between two surfaces that concave rather than convex.

Easy to use the fillet tool to create an outward curve but how can I reverse it so it becomes inward?

Thanks!

It's not as simple as it looks at a glance, because it splits the edge, offsets the surfaces' edges, inserts two new surfaces and then fillets the new edge. So, the simplest way I know is to fillet the edge needed, then copy the object, convert the copy to surfaces and delete the fillet surface. Thicken the surfaces adjacent and add them so that they give a new edge that can be filleted. The other methodology is to copy the object, fillet the edge normally, then convert to curves, then use the fillet's edge curves as rails for an inverted profile.

The most useful methodology is to thicken the surfaces adjacent to the edge you want to inverse-fillet. Intersect them and it gives a collar that can have a appropriate edge treatment applied.