You mean the stretching? This is caused by the UV map and can not sufficiently be recalculated without distorting neighbouring terrain patches. But you could use a ttriplanar UV mapping with a shader to calculate texcoords by slope, see this: http://memoirsofatexel.blogspot.de/2010/08/terrain-triplanar-uv-mapping.html or http://devblog.drheinous.com/2012/06/triplanar-texturing.html or http://http.developer.nvidia.com/GPUGems3/gpugems3_ch01.html.

If you are working with a blendmap, you need to update a texture. For this you can use bmap_process to apply changes, or you change the bitmap with the pixel functions, which is kinda slow. Basically, all you have to do per frame is

1.) to convert the mouse cursor position from view space to object space
2.) convert that from object to uv space as offset for the drawing
3.) iterate over all pixels: for each pixel calculate strength and update

in your multitexturing shader you have to use this bitmap then; you can either update the entity's texture of the blendmap or you create a dynamic blendmap on the fly and assign it as material bmap on skin 1 or else.