Cokin Sunsoft filter
 

I was wondering if this blocks enough light to increase the aperture size so I can get a more narrow DOF kind of like an ND filter.

I've got a screw-on Cokin Sunsoft in the 43mm thread size. It does block some light, how much, I don't know. Perhaps as much as the weakest ND? Maybe not even that much.

Okay, I checked the Cokin Booklet. It says, Filter Factor 1.3, F-Stop 1/3. (To compare this with a Cokin Grey ND 2X, the Filter Factor is 2, F-Stop is 1.)