I bought a 4 yo gray filly (gray on both sides of her pedigree - all four grandparents are grey), but I know her dam and a bunch of her half siblings. The dam is 25, has no melanoma, and is living her best life as a retiree. Of her half sibs, only one has any melanoma (currently the underside of his tail) and it hasn’t caused any issues. That half sibling is 21. The other half sibs that I know are 9, 8, 6 and 5 - no melanoma.
So I took the chance, since she was well known to me and I was sure of her suitability.
In your situation, I think I would have passed too.
Sorry, but I think you made the right decision.