Flask discolouring scotch...

[Disastrous]

I was gifted a hip flask on Saturday evening and christened it with some Talisker. Didn't finish it on the night and irritatingly forgot about it until now.

I poured it back into the bottle and too late, I realised it had discoloured and blackened slightly!

I know they say not to store booze in a flask but what causes the discolouring and is my talisker now poisonous, or worse, ruined??

[Poisson96]

Being a drinking vessel, I doubt it would be a poisonous substance

[55palfers]

Is it a pewter flask?

[Disastrous]

Stainless steel...

I presume the whisky has started off some oxidisation or somesuch but no idea really!