What's different between empty values and 0 values.

The main reason that you are seeing levels tracked into Cue 6, is as you say, you are editing with units that have never had a value before, including a zero value.

You need to use Block or Intensity Block on Cue 6 to ensure that Cue 6 will forever be a blackout, regardless of what happens upstream of it.

Syntax is - Cue x Block Enter or Cue 6 {Intensity} Block Enter.

This will place a 'B' Flag in the Block column of the Playback Status Display (PSD). Showing you that this is now a blocked cue. (The 'B' is different to a 'b' in this column, as it shows that it is a Full Block on the Cue, rather than a Partial Block 'b', ie only certain channels blocked. Which you are probably seeing at the moment).

The concept of blocking is that it is like building a wall, where nothing can penetrate into the cue from upstream in the cue list. Therefore a general rule of thumb is to always Block blackouts, big look changes, start of new Scenes etc etc. Anywhere that you know that the 'look' is the starting or end point, and you don't want edits to get into it.

The difference between a Block and an Intensity Block is fairly self explanatory, an Intensity Block will only block the Intensities, allowing NiP's to track values through, this can be very useful in certain cases, where you might change colour and see your scrollers whizzing back to the original colour while fading in your Blocked Blackout! An Intensity Block would allow the colour information to track into the Blackout. 

Note - Blocking is only a programming level command, it does not affect playback. Therefore watch out for Snap Blackouts crashing on top of long running cues. You will need to read up (or play!) with Assert in this instance, to ensure that your Blackout is not only always a blackout (Block) but will always be a Snap too (Assert).

Hope that makes sense.

Warren.

Short version:

"Null" or no value can be overridden by any other source. As you note this includes tracking into a cue.

0 is a hard value like any other value. All the fancy HTP/ LTP/ assert/ priority rules apply.

To get around this, some people choose to record a "zero" cue at the beginning of a cue list, basically setting everything to a hard zero so the zeroes will track forward through the cue list. This won't really work in situations where you're adding onto looks by playing back multiple cue lists simultaneously, but it works well for shows that are run in sequence with one main cue list.