Replace values from one column to another based on a condition

Hello!  You were pretty close it.  Instead of searching for blanks you would want to search for null.  Also, Replacer.ReplaceText will replace substrings; Replacer.ReplaceValue will replace the entire cell's contents.  So there are two options....

This first option uses the coalesce operator (??) which will simply return the first value that is not null out of the options you provide it:

Table.ReplaceValue ( #"Previous Step", each [Col A], each [Col B] ?? [Col A], Replacer.ReplaceValue, {"Col A"} )

The second option is to use if/then:

Table.ReplaceValue ( #"Previous Step", each [Col A], each if [Col B] = null then [Col A] else [Col B], Replacer.ReplaceValue, {"Col A"} )

Complete example:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQ0MDbUUtKBMJRidYBCpkaGZmAhEAMiZGBgABQAs40NzMwUgv38gXwQEyZvaIxQYGlhaQjkGVkaAenYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Col A" = _t, #"Col B" = _t]),
    Custom1 = Source,
    #"Replaced Value" = Table.ReplaceValue(Custom1,"",null,Replacer.ReplaceValue,{"Col B"}),
    Custom2 = Table.ReplaceValue ( #"Replaced Value", each [Col A], each [Col B] ?? [Col A], Replacer.ReplaceValue, {"Col A"} )
in
    Custom2