[msl]

I set up a cell builder and moved it to the house for ease of use, left a large split behind.
The split had capped 10 cells on day 5, came back day 10 and the cells were tore down, one left that that turned out non-viable. Gave them a bar of brood from a overwintered hive created form this hive last year, came back and nothing.

Did a fly back split on the overwintered hive and gave them another bar of brood, came back to day (10 days) to make splits and there was nothing, found one cup with what looked to be 5 day old larva in it with dark (grey )stripes and a bunch of empty cups

The cell builder hasn’t turned out a viable. cell yet, lost 4 rounds. I had blamed the incubator as I found it spiking to 100, so I left the cells to be finished in the builder, took a peek and the bees seem to be ignoring the cells tore a few open and they still look ok, figured will know in a few days

But now I am not so sure I don’t have another issue as well.
Hives have sold brood pattern. In the incubator tip of the cell is turning black, bunch of “goo” in the tip. I get the impression the larva/pupa is dying falling to the bottom and rotting. Doesn’t sound like what I have read on BCQV.

 

[Apis Natural]

My first thought after reading was you have bqcv.
After some reading here in the office it's quite possible. My book on bee disease says "Once the larvae are black, you may notice that the outside of the wax queen cells will have a black “oily” spot on them." that could be the goo you are seeing.
Have you opened the cell to look inside?
make sure to use a tool that you can sterilize or throw away so you don't cross contaminate.