use BlankZero with another value<>0 in pages

BlankZero property does exactly what it says on the tin = blanks zeros, and zeros only

If numbers allowed in your field are non-negative (0 or bigger) you could use BlankNumbers property and set it to BlankNeg

If the field is supposed to display all negative, zero, and positive numbers, and only this one specific value -9999.999 is supposed to be hidden then you need to write a bit of the code to manage it - a function or two - depending if the field is supposed to be editable or not.

If the field is non-editable then you would only need one function, like this:

BlankMyNullValue(InputValue : Decimal) : Text
BEGIN 
  IF InputValue = -9999.999 THEN
    EXIT('');
  EXIT(FORMAT(InputValue);
END;

You could use this function directly in SourceExpr property:

SourceExpr:= BlankMyNullValue("My Field");

If however you need to keep the field editable then you would need to created global text var on the page, assign the var to the SourceExpr property, in OnAfterGetCurrRecord trigger set the value of the field using SomeGlobalTextVar := BlankMyNullValue("my field"), and in OnValidate you would need another line of code to interpret what user typed in the field and assign it back to your field.

OnAfterGetCurrRecord()
BEGIN
  SomeGlobalTextVar := BlankMyNullValue("My Field");
END;

The Field - On Validate()
BEGIN 
  EVALUATE("My Field", SomeGlobalTextVar); 
END;

The drawback is that once you start using code to manage displayed values you will lose the possibility of sorting along that field.