The bonnets are black because of the material being used. Mostly
are a composite material made of carbon fibre or a mix of kevlar
and or carbon composites.
Being made of carbon fibre, it is usually much lighter than the OEM
metal bonnets. Being lighter, it means your car's power to weight
ratio increases thus increasing the car's performance.
What you see on the streets are nothing but a trend going around
now. There are much more things that you can do to your engine and
transmission before doing the carbon fibre bonnet bit. But you cant
see engine mods as clearly as you can see a black bonnet. Some do
it for cosmetics while others serious into racing do it to lighten
their car and thus improving the performance. Usually, one would
put bonnet pins onto the carbon bonnet to prevent the bonnet from
unlatching and flying off during high speeds.
hope this has helped.
