Not suer where negative associations came from.
Just to point out second word in my reply is guess. And as time goes I can say educated guess as I and few of my friends asked the vendor about origin of the hones/material. There would be appropriate to use derogatory term for quality of those answers.
Dictionary says "Slag heap"- In quarrying- term used for a hillock of disposed waste material from quarrying. Nothing about derogatory term. Not sure how you translate "slag heap in in Bethesda" to Welsh language but I am sure it is not "Reclamation yard in Bethesda" which is term used by vendor to answer when people asked where is the material from. I was nicely laughed at by my friends (who live in Bethesda) when I asked them about the reclamation yard where you can get slate. Once you would be there you will know why.
Here you have picture and you can find all colours of slate there as it was quarried from different depths.
53.170084,-4.059191 (Welsh Slate) - Google Maps
In any of my previous replies about these hones I did not deny that they can be decent hones. I only do not like when established names of hones and knowledge about them is being marginalised by the vendor for his commercial gain.
Would you call the car in the link bellow Ferrari just because it came from a car manufactory, it is red and has a horse in it?
Google Image Result for http://img364.imageshack.us/img364/9466/farari13wc.jpg