Texture borders

I have a very huge texture which is too large for many graphics cards. That's why I wanted to split it into multiple textures and then tile tile it togeter. At the border of two tiles you could notice the change of the texture because I am using linear filtering. That's why I decided to add a 1 pixel texture border to each tile texture. Now I've created the texture border but strangely OpenGL always uses a completely black border for filtering. Setting GL_TEXTURE_BORDER_COLOR to a different color doesn't change anything, so I've no idea from where it takes the black color. Any hints welcome.