Blacks become illegal after uprezzing from 720p to 1080

So I've got my 720p timeline in FCP with my finished color graded clips with 2:35 aspect slugs on the top and bottom of picture (on separate video tracks)...I put on an RGB limit filter to clamp my blacks at 0 and everything looks great for Broadcast Safe, according to my FCP scopes anyway. But then when I export and uprez to 1080 in compressor and bring it back to a new 1080 timeline my blacks show traces well below zero, mostly in the blues! Anyone else ever have this problem?

A little more info: Could bit-rate in video processing be part of issue?

I exported my original DVCPROHD clips into Color, graded, then output as ProRes 422 clips at 8-bits (since they obviously went in at 8-bit being DVCPROHD)...and my video processing tab in FCP is 8-bit YUV...but even when I try it with 10-bit in FCP I still get the same results...don't know if this is part of my issue or not?? I switched some of my Project Settings info in my 1080 timeline but still not having luck...Is it Compressor that is giving me issues on uprez? Seems like it's FCP...the 1080i timeline that is...