No this is not possible.
If you do want to do this, you would need to write your own function. For example:
Function COUNTSHADED(rng As Range) As Long
Dim lngCount As Long
Dim CellA As Range
For Each CellA In rng
If CellA.Interior.Color <> 16777215 Then 'Blank interior
lngCount = lngCount + 1
COUNTSHADED = lngCount
You can then use "=COUNTSHADED" function like any other function.