You can simply `Reverse` your label list and use `"ReversedRow` layout without having to process the color scheme:

    SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
     LegendLayout -> {"ReversedRow", 1}, 
     LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
     LegendMarkerSize -> 30]

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/c35hA.jpg