The adjectives in French change the form to correspond the gender and numbers of the noun that they modified. Therefore, you can say :

Elle a les cheveux blonds …. (1) or Elle est blonde…(2)

Notice how the adjective “blond” change its form in the above sentences. The noun in (1) is “cheveux” which is masculine and plural, while the noun in (2) is “elle” which is feminine and singular. Therefore, the adjective “blond” become “blonds” (masculine, plural) in (1) and “blonde” (feminine, singular) in (2).

I have summarized several rules when changing the form of an adjective from masculine to feminine:

1. when adj ended in “e“, there is no change at all. e.g. Tranquille, terrible, rouge, jaune, enthousiaste

2. when adj ended in “f“, it becomes “ve“. e.g. Sportif (Sportive), créatif (créative), crantif (crantive)

3. when adj ended in “eux“, it becomes “euse“. e.g. Chanceux (chanceuse), ambitieux (ambitieuse), studieux (studieuse)

4. when adj ended in “ier“, it becomes “ière“. e.g. Dernier (dernière)

5. when adj ended in “ien“, it becomes “ienne“. e.g. Italien (Italienne), Canadien (Canadienne)

6. when adj ended in “ain“, it becomes “aine“. e.g. Américain (Américaine), Mexicain (Mexicaine)

7. when adj has a consonne, e.g. Gros (Grosse), Bon (Bonne)

8. Generally, add a “e” at the end of the adj to form the feminine form.

Of course there are exceptions, like Vieux (Vieille), nouveau (nouvelle), beau (belle). More about the plural form of an adjective, read here.