Formula to find last different value in a column?

I have a formula that is based on comparing the last value in a column.
Basically of not null it performs an action.

However I need the ones after that to be able to set up and find the last
different value e.g.

A
B
B
B

My formula so far is if Value"" then do X
I even got it to say If Value"" AND Value not equal to prior record.
Which would work for the second B

But what I really need it to say (to get to A) is
If Value "" and Value IS Equal to Last Record step backwards until you
find the value of the first different record.

Is that possible?