Blackstone develops it's universal averages from each engine series, so the numbers you see are indeed fair and accurate.
They do NOT homogenize the data across all submissions; they are uniquely tailored to each engine family. As an example, when I see the universal averages in my UOA for my Durmax 6.6L in my Chevy truck, those universal averages are specific to all vehicles that run only that 6.6L diesel engine; they are not crossed into other diesel engine results. Same goes for the 6.0L PSD, versus the 7.3L or 6.4 or 6.7L Ford diesels, nor the 6.7L ISB. Each "universal average" exists for each engine family.
Now, in regard to the Vortec engines v-8's (4.8, 5.3, 6.0) I'm not exactly sure if they sub-divide down in regard to displacement, or just average across that engine family line, but since those engines are basically bored/stroked versions of each other, there is good cause to see similarities.
You need to know how your exposure (mileage) compares/contrasts to the "UA" of the UOAs. Did your sample run at or near the same at the universal exposure? Typically Blackstone will indicate what mileage those UAs are based upon. If you are near the UA mileage, and your Cu is 59 when the "norm" is 29, then you indeed have a high Cu reading. It is true to say that many GM products have higher Cu readings, but that is NOT an automatic license to just ignore anything that is above unviversal average, because that universal average already takes into account the general metal counts for your specific engine family.
That should not be an indication to panic. It is cause to contine to monitor conditions, and stay as consistent with your inputs as possible, so that shifts in ranges or trends will help you realize what's truly going on with your lube selection and maintenance program.