This one supports more tokens (173) but uses $lf as separator, which is uglier:
Code:
alias blink {
  var %n = $regsubex($str($+($,a,$lf,$chr(44)),$0),/a/g,\n)
  if $0 && $count($($!+( %n ),2),$lf) == $0 { echo -a $* }
}
Based on the fact that if $N is $null, $N<anything> becomes $null too (and assuming no $lf is in the params).

Btw, you need $istok() instead of isin (for cases like $len($5) = 10).