Combine the fish stock, wine, shallots, garlic, and thyme in a medium saucepan.

Bring the mixture to a boil and cook over high heat until reduced to a syrup-like consistency, about 12 minutes.

Remove the pan from the heat and slowly whisk in the butter, bit by bit.

Add the blackberries and heat the sauce through over low heat. Set aside and keep warm.

Steam the Mahi-Mahi fillets in a covered steamer over medium heat for 5 to 6 minutes (depending on thickness), until the Mahi-Mahi is firm and barely opaque in the center.