Replace blank in single csv column throughout multiple files

Question

I am trying to sift through a batch of CSV files (using | as delimiter but prepared to replace pipes with commas if needed) replacing any blank field with another value using PowerShell. The CSVs don't have headers, so I need to add headers to focus on column 3, but the file written back to disk needs to not have these headers.

I've tried the following with no luck. It seems to do nothing:

  $files = Get-ChildItem "~\Desktop\Test\*.csv"
foreach ($f in $files)
{
    $outfile = $f.FullName + "mod"
    $data = Import-csv $f -Header A , B , C , D , E , F , G
$data |
   ForEach-Object {
$_.C = $_.C -replace "" , "BLANK"}
}