Been reading, and re-reading, this thread with interest. First let me say I am not generally a baseball collector and I have never owned a '58 Aaron, so I cannot comment on other peoples card that they have in their hands and can see better than I.
Having said that, I am generally a skeptic, and here is why.
Doing some general research on printing, I have found out that this type of printing is referred to as CMYK printing (C=cyan, M=magenta, Y=yellow, and K="key" or black). It is my understanding that printing is generally done in the order of the initials. In otherwords, a sheet is printed with cyan first, Magenta second, yellow third and finally black. There are a couple ways I could see a sheet getting through without the yellow being printed.
1 - The press ran out of yellow ink.
2 - An operator removed the sheet after the magenta printing, skipped the yellow and put it back on the press before the black (not likely in my mind).
3 - 2 consecutive sheets could have got stuck together after printing the cyan and magenta. If stuck together, the top sheet would recieve the yellow, but not the bottom sheet. This sort of thing happens, I'm sure, however it doesn't seem likely that the sheets would separate after the yellow and each make it through the black process. I would think if the sheets got stuck together, the bottom sheet would miss the yellow and the black.
Regardless if any of these happens, the fact remains that the entire sheet would be missing the yellow.
Here is a picture I found on the internet, of a partial(?) uncut sheet of 77, 1958 Topps baseball cards, that includes the Aaron card.
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If a single sheet, assuming a sheet only consisted of these 77 cards, made it through without the yellow, then not only would there be a blue Aaron card, but the 32 cards shown that have a yellow background, would in fact be white. This would include Ted Williams, Duke Snider and Don Drysdale.
My own opinion is that most are sun, or light faded, but I don't mean intentionally.
I have seen numerous card dealers over the years, both in shops and open-air sellers, who have had the same stock of show case cards for years. Cards that sit in showcases, without being rotated or sold, will, over time, fade from the sun or even fluorescent lights.
the Aron card is a key card in the set and would have been more likely to be in a showcase than other "common cards from the set, thus it is more likely to get faded.
Does this mean that all are illegitimate? Of course not. Anything is possible with Topps' suspect quality control.
Looking at some of the "blue" Aarons I can find on the net, it is obvious to me they are indeed faded. You can still tell the white around the Braves logo has yellow tinge to it, compared to the white border of the card.
Just my 2 cents worth.