Return non-blank cells from range AND ignore cells with only formula w/o results

Hi. I'm rather new to excel, and am having trouble merging two formulas.
In column A I tick off items using different letters according to their category, and my existing formula in column D (for simplicity's sake, something like IF A2=x, F2)pulls the associated order numberand places it in the D cell.

I want my formula on a separate sheet to return all the non blank values in the column, which is easy enough using this formula posted in another thread on here
=IFERROR(INDEX(D:D,SMALL(INDEX(NOT(ISBLANK($D$2:$D$20))*ROW($D$2:$D$20),0),COUNTBLANK($D$2:$D$20)+ROW(D1))),"")
This however is not ignoring blank cells as opposed to empty cells.
edit: if that's not clear, it does ignore truly empty cells but if there is a formula in the cell, even though the formula is not returning any value, it sees the formula and counts it as non blank.

I found this on another forum, which works in its own right but I am unable to merge the two correctly.
=IF(A1="",empty_situation,non_empty_situation)

I'm sure there's a plainly obvious solution I'm overlooking but would appreciate any help.

Why not create a pivot table with the range being only Column D?

Then add the field to the row section of the pivot. It will desplay all items, and you can tick off the blanks in the field options

If you want all non blank + Unique values for a specific column, the Pivot is the best way to do it.
No need to add any extra columns. Just create the pivot table with 1 column as your data source and tick off the blanks you don’t want.
The added bonus of the pivot is that as new data arrives, the pivot updates also.

It seems though that what you are looking for is
The unique list without blanks from 2 Columns.

Column B and Column C. For this you need 2 pivots

Create 2 Pivots 1 for column B and 1 for Column C

Column B data Source B14:B20000 - Put the field in the row section and tick off blanks
Column C data Source C14:B20000 - Put the field in the row section and tick off blanks

Alternatively you can add an extra field to your "data" sheet where you put an extra columnO and insert there the X or A marks.
Your original pivot table will now have the "A" or "X" marks as a field with no need to have those columns B or C with the if statements

Glad I could help.

You can save a bit of time also if you create a small macro to update all the pivots automatically by pressing an update button.

The code is not that hard to do and you will make sure all your date is up to date very quickly.
If you need help with that start a new thread in the macros/vba section.