I know the OP said he or she is looking for open heel fins, but I just wanted to say that I have Atomic Blades and Hollis F-1s and I got a set of the Mako freediving fins at Thanksgiving. I used them for a week in FL and just got back from a week in Hawaii. And the freediving fins are now my "go to" fins for any warm-ish to warm water diving (that does not include going inside a wreck).
They are awesome. The thrust for any kind of kick leaves my other fins (which are dang good fins!) in the dust (silt?). Even for "technical" kicks like frog kicks, helicopter turns, and back kicks, the freediving fins work better for me than either of my other sets of fins.
And being full foot means I don't have to wear any kind of shoe or boot, if I don't want to (though I have been wearing Lavacore socks).
The one sorta negative point to make is that they are so long they don't really fit very well at all in my big gear bag. For my flight to Hawaii, I disassembled them (meaning, removed the blade part from the foot pocket part) and then they fit in my gear bag just fine. For that reason, I would be very cautious about buying a set of the Mako fins that don't have replaceable blades. I'm not sure if these Los Ninos fins do or not. The ones I got are the "Competition Freediver" and they do, but the less expensive freedive fin they have (for adult men) do not have replaceable blades, I don't believe.