Just a paint drop from a syringe. Actually, one of a couple hundred probably. I now have an even greater appreciation of IreneM's work!
PP:
Channel Mixer (shot as red, changed to blue as I thought it more interesting), Slight Rotate, Brightness/Contrast, Noise Reduction, Crop, Clone out minor blemishes, Size & Add Border.
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The bottom left of the image is a tad blue, which I'm guessing is caused by reflection of light off the (paint?) and onto the background. You'd do well to clone it out to white, because its pretty distracting left as-is.