Brun/Brune are the masculine and feminine for brown in French, Brunet/Brunnette are the masculine and feminine diminutive Ike a cute way of saying something- like “Michael” vs. “Mikey” in English).
Brunette/Brunet can actually apply to a wide range of hair colors, including black hair, and even gray hair which may seem odd, but the root of the French Brun is “bruhn” which is a proto European in word for brown/gray. You have to remember that color concepts and terms have evolved over time and most of the time, historically they often didn’t have the same colors as words or concepts as we do now, with “blue” being a classic example of a color that in many languages didn’t get its own word until relatively recently in human history. The limitations they had in access to dyes and various other factors likely contributed to this further. So blond/blonde,brunette/brunet, and red head or ginger etc. have most often been the three classifications of natural hair color.
Under light, “black hair” can often be seen to actually be brown, and one can have blonde or brown hair with reddish tint to it. You can also have light brown hair or “strawberry” or “dirty” blond hair- so we can already see where the concept of classification of hair into these sorts of broad groups can be subjective when done as hoc. What one considers light brown another may call dark blonde for example, with words like “flaxen haired” often used in English to describe a certain range of pale yellow to yellow ish brown hair. The word entered the language after a time but didn’t always exist as a concept, and in modern times the usage is very uncommon.
Confusing the issue further, blonde derives from blont or blund in old French, which is debated to come from either the Latin blundus- yellow, or the Frankish root referring to gray hair.
In the nearer root, the old French, blund referred to a color that was between golden and chestnut, which in modern times we might refer to a chestnut color as a light brown or as a chestnut blonde.
In usage, blonde became a replacement for “fair” so in theory, blonde can refer to any lightly pigmented hair including white hair and linguistically white hair would be blonde and gray hair would be brun in old terms, however this is subjective and influenced by color concepts of a period.
We can see this with blonde itself- which for a time in various parts of Europe didn't only refer to hair, a blonde was not simply a person with yellow hair, but due to replacing the word “fair,” blonde referred to someone who had both fair hair and skin, and could even be used for one or the other in certain cases.
So much like other subjective and imprecise concepts like race, the use of terms like “blonde” or “brunette” aren’t very precise and cannot be relied upon to have any clear and standard meaning across speakers and across time.
To black haired people specifically, if we follow the conventions of blonde and brunette in using the diminutive of the French root, one proposed term would be “noirette” as the feminine, like blund and brun, noir is a color coming from French and in this case, specifically referring to black. Noir comes from around the 12th century from the old French neir which is from the Latin “negro” meaning black as well in Latin.
So “noirette” would be the closest linguistic match the blonde or brunette.
In French it isn’t common to use those words though- a “brunette” would simply be referred to as their gender and brun/brune to match- so like “un garçon brun” with garçon meaning something like boy in English and brun being his hair.
So a black haired woman wouldn’t be called a noirette in French most likely either, noiraude or the masculine COULD be used but most likely you’d just speak similarly to the example for brun above or a very text book French way like: “Elle a les cheveux noirs“ which would literally be more or less “she has black hair.” Like many examples of text book language, it isn’t necessarily common to speak this way especially informally.
Lastly- if we deviate from the French root, Gaelic has “Brenna,” or the masculine, which basically means “raven/black haired” so that could work too, and we can borrow words or phrases from many other languages too.
Brunette/Brunet can actually apply to a wide range of hair colors, including black hair, and even gray hair which may seem odd, but the root of the French Brun is “bruhn” which is a proto European in word for brown/gray. You have to remember that color concepts and terms have evolved over time and most of the time, historically they often didn’t have the same colors as words or concepts as we do now, with “blue” being a classic example of a color that in many languages didn’t get its own word until relatively recently in human history. The limitations they had in access to dyes and various other factors likely contributed to this further. So blond/blonde,brunette/brunet, and red head or ginger etc. have most often been the three classifications of natural hair color.
In the nearer root, the old French, blund referred to a color that was between golden and chestnut, which in modern times we might refer to a chestnut color as a light brown or as a chestnut blonde.
In usage, blonde became a replacement for “fair” so in theory, blonde can refer to any lightly pigmented hair including white hair and linguistically white hair would be blonde and gray hair would be brun in old terms, however this is subjective and influenced by color concepts of a period.
So much like other subjective and imprecise concepts like race, the use of terms like “blonde” or “brunette” aren’t very precise and cannot be relied upon to have any clear and standard meaning across speakers and across time.
So “noirette” would be the closest linguistic match the blonde or brunette.
So a black haired woman wouldn’t be called a noirette in French most likely either, noiraude or the masculine COULD be used but most likely you’d just speak similarly to the example for brun above or a very text book French way like: “Elle a les cheveux noirs“ which would literally be more or less “she has black hair.” Like many examples of text book language, it isn’t necessarily common to speak this way especially informally.