Creating a field of darkness

I want to make a field of darkness that vision can’t penetrate but the players can walk through but won’t see anything. Like powerful magical darkness 

I would use a blacked out token for that, or two, one for square and one circular. Place them on the token layer, drag to size and move to the front. All other tokens will be under it.

You can do what Axecleft said on the token layer. Then if you want to block line of sight as well, you'll need a polygon on the dynamic lighting layer traced under it. Finally you will need to uncheck the "Restrict movement" box in the Page Settings. This will mean players can walk through ALL dynamic lighting lines, but there's currently no way to designate individual DL lines as movement/sight restricting. Like this, but I'm sure you can make a much better polygon: Drawbacks are having to move the DL polygon and black circle token separately, and trusting your players not to go running off through all the locked doors while you have movement unrestricted. edit - I'm sure I've seen a script around that mirrors token movements... whether or not this can be used to mirror token movement to a dynamic light layer shape I do not know

This would be easy to do if regular Fog of War had "Polygon Hide" in addition to "Polygon Reveal". As is, you could create an FOW square and use "Polygon reveal" to carve away at the edges to make it circular. Andrew's method is prettier...