Welcome and thanks for posting your query to Healthcare Magic.
As per your problem you are Suffering from Raynaud's type reactions since long time and today you felt black bruise
on hand suddenly.
As per the query posted by you I can get that you very well know about Raynaud's reactions. It is quite common tin some persons that after exposure to cold there is progressive redness
and then bluish discoloration of the skin.
This process is due to cold injury
to the blood vessels which are contracted because of chilling effect of cold. This is also to conserve the heat inside. Now in your case today you felt the blackening of skin means this time the cold injury was too high and your skin was not able to mange the constriction of vessels. SO the blood vessels got very much contracted and lead to blackish discoloration.
Because of the wise thing of putting warm water on the area, it again regained its blood supply.
Now generally it can be due to the intensity of the cold but if you feel it even with simple cold water or recurrent episodes of blackening in future then it is better to consult a dermatologist
for best further guidance.
Hope these informations will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)