Yes, with oily rags, the right light, and soot, the grey could at times look (especially in black and white photos) as black or blackish.
I saw two pics one time of a J15 on a train - possibly an enthusiast's special of some sort if memory serves me right - both taken of the same train in the same station on the same day; and by the same photographer. One, with the light right, is clearly grey, albeit dirty. The other side of the loco with the light a different way looks distinctly black! GNR blue was another colour which could look lighter at times, giving rise to the quite incorrect myth that Dundalk "just went down to the nearest paint shop".
I actually knew the former Dundalk Works Manager very well, and he found that enthusiast-fuelled myth quite offensive, pointing out that they had their own paint laboratory in the works.....
CIE green - particularly the later, lighter shade is notorious for appearing in numerous apparent varieties in olour photos. I am currently going through a collection of these for a future book, and in one photo I have, three green vehicles look, respectively, accurate green, navy blue and brown! ALL are CIE green!