There is no such property as:
.Font.HighlightColorIndex = wdBlack
Moreover, you can't set the HighlightColorIndex that way via Find/Replace. You need to use something like:
Code:
Options.DefaultHighlightColorIndex = wdBlack
With ActiveDocument.Content.Find
.ClearFormatting
.Font.Shading.BackgroundPatternColor = RGB(235, 214, 214)
With .Replacement
.ClearFormatting
.Font.Bold = True
.Font.ColorIndex = wdWhite
.Font.Shading.BackgroundPatternColor = RGB(0, 0, 0)
.Highlight = True
End With
.Execute FindText:="", ReplaceWith:="", Format:=True, Replace:=wdReplaceAll
End With