The black bottom is a reaction of residual fumes from the flame, combined with the heat reaction of the metal. This isn't a dangerous sign at all.
Depends on what the "speedos" were made of - natural fabrics or man-made.
A black surface absorbs heat better than a shiny one. However, it's not likely to be simply painted, as heat would be likely to damage paint. Many cooking utensils are silvery because they are made of metal which is a good conductor of heat.
Carbon Fiber. It's often left unpainted, which gives it a beautiful shiny grey-black metal color.
Arsenic is shiny,gray,brittle metallic looking solid.The dull coating is made of arsenic oxides.
This could refer to the colors that are made to use with a black light. Colors that glow under a black light.
Yes, cooking oil is made out of oil.
A common alternative ingredient to create a shiny finish on baked goods instead of egg wash is milk or cream.
The coin was likely made of copper, which is an element commonly used for making coins. When heated, copper reacts with oxygen in the air to form copper oxide, which is a black compound.
theres no shiny stone on ruby because when the game is made the shiny stone doesn't exist yet.
Wax
Purely aesthetics these days, to match other kitchenware. Increasingly kettles are made of plastic.
JK Rowling never gave any details about Snape's wand. In the film, it is black and shiny with a decorated handle.