gistfile1.py

# my_page.models.py
from mezzanine.blog.models import BlogCategory

class MyBlogCategory(BlogCategory):
  color = models.CharField('Hexadecimal color', max_length=6, blank=True, null=True)
  class Meta:
    verbose_name = "My Category"
    verbose_name_plural = "My Categories"

# my_page.admin.py
from copy import deepcopy
from django.contrib import admin
from my_page.models import MyBlogCategory
from mezzanine.blog.admin import BlogCategoryAdmin

category_fieldsets = deepcopy(BlogCategoryAdmin.fieldsets)
category_fieldsets[0][1]["fields"] += ('color',)

class MyBlogCategoryAdmin(BlogCategoryAdmin):
  fieldsets = category_fieldsets
  list_display = ('title', 'color')
  
admin.site.register(MyBlogCategory, MyBlogCategoryAdmin)