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Completely blue fabric shoots blue and appears blue for the first few moments then this dark blue/ black appears.
Persists after jumping to Photoshop, persists on export, dissappears if I edit overexposed.
There are no changes to curves or colours beyond white balance.
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As I assume you know, for RAW shots, the first image you see in the filmstrip, is from the embedded JPEG, Lightroom converts that RAW data and creates the initial preview, typically showing you something not as you remember shooting, not as seen in the viewfinder.
So a bit of post processing.
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I notice you are going with the default profile Adobe Color. recommend you change that to something more to your liking, probably a camera matching one. Works wonders. This is tyically the first edit I make.
I never ever can truely trust that Lightroom interpetation of WB, As Shot. might be a bit OCD of me. that is usualy the second edit I make
You stated "There are no changes to curves or colours beyond white balance." Okbut it looks like some general edits were taken, just not WB and curves. Did you perhaps attemt Auto Tone?
Getting at third step I would take, some will highly disagree, I would hold down on the shift key, and double click on the Whites slider, then repeat on the blacks slider, and perhaps the misnamed Exposure slider (others well screame NO... at that bit), a way to auto set my black and white point. Tyically I do not like complete Auto Tone (although it has improved)
Then go and select a curve.
That sets me up for my start point for more artistict editing
And then move on.
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Thank you but there's no artistic editing here! it's standard ecommerce at the same start point as 200 other products on the day. It's behaving a bit like a moire pattern problem, only comes in as I adjust the exposure or colour and like a preference problem in that it only comes on after I let go of the slider and move to another task. Only on this colour, not an overdye issue. Only happening in Adobe.
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What does a straitght out of camera jpeg image of it look like?
This looks more like some discolouration of some sort in the fabric or diff in reflectivity??.
Possibly made visible to the camera due a UV filter vs. non-visible to human eye?
I would not expect other artifacts etc to maintain the full texture of the fabric.
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Here's the side by side with the raw file in finder.
 
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Looks like moire' interference patterns. Try applying the Moire' control to the whole image using a Graduated Filter dragged off the canvas as shown below. If that doesn't fix it please export one of the CR2 files to DNG file format, upload to Dropbox or other files sharing site, and place the share link in a rpely here. Thank you.
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Very much like a moire pattterm, and thank you for taking the time. 
 Filter left to right
Just in case, right to left.
Here's how it looks as you actually drag the slider
 
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I downloaded the DNG file and this is not moire' interference. It appears to be due to the camera profile with Adobe Standard exhibiting less than Adobe Color, but both exhibit the shift blue to black in some areas of the fabric.
I created a Canon 5D MKIV Adobe Standard legacy camera profile about a year ago that exhibits very little of the blue to black shift. You can download and install it as outlined at the below link. The rendering is very similar to the new Adobe Color creative camera profile.
EDIT: If interested here's a link explaining how I created the Adobe Standard Legacy camera profile. It is applicable to ALL camera models introduced starting in September 2014.
I suggest reporting this issue in the Photoshop Family forum that is monitored by Adobe staff at the below link.
https://feedback.photoshop.com/photoshop_family
You can reference this post in your report and then place a link to it in a reply here for others with similar Canon 5D MKIV issues.