I seem to be having a problem applying grass, dirt, rocks and so on. Whenever I use this feature I get (sometimes) black spots that show up instead of the grass. It seems to be happening more often the more I use it. Right now I have a spot I can’t seem to paint over and I can only use about three options for landscape. I remember seeing this problem a few years ago when I was modding Oblivion, but can’t remember what I did to fix it.
Any ideas on this would be much appreciated.

Each cell is divided into 4 quadrants. Upper left and right. Lower left and right.

Each of these quadrants can contain a maximum of 6 landscape textures. If you try to paint with a 7^th texture, it will paint as black colour over the existing textures.

What you have to do is decrease the amount of textures in the quadrant. You can do this by making sure your view point is in the the cell and hit i when the Render window is active (you must also have the landscape editor open). This will bring up another box showing you which textures are currently used in each quadrant for that cell. Take a note of the higher percentage textures. Right click on one of the lowest percentage textures and select Replace. Scroll the list and select one of the higher percentage textures. This will then replace the lower texture with the higher texture and reduce the texture count in that quadrant by 1.

Example:

Upper right quadrant contains 6 textures

Grass 1 - 40%

Grass 2 - 25%

Grass 3 - 17%

Grass 4 - 13%

Grass 5 - 7%

Grass 6 - 3%

Right click Grass 6 and select Replace. From the list that appears, select one of the higher percentage grasses. Suppose you select Grass 3. The quadrant will now show -

Grass 1 - 40%

Grass 2 - 25%

Grass 3 - 20%

Grass 4 - 13%

Grass 5 - 7%

and this allows you to add a 6^th texture of your choise.