When blue-water paddle warships reached their peak

Were the 1840s the high-water mark of blue-water paddle ships?

No. Their peak came closer to the 1860s, when paddle designs were still developing.

The first great burst of paddle-steamer adoption did not mark the technology's final form. Blue-water development continued into the 1860s. That date matters because it overlaps the rise of screw propulsion rather than preceding it cleanly. The two systems coexisted and developed in parallel before screws decisively captured the ocean-going warship role.