I see you have huge terrain, so with one blend map you then need very high resolution texture for result to be somewhat acceptable, apart some other issue that will occur. All in all i would not recommend that approach.
When you partition terrain mesh you also partition blend map into multiple textures that
tile/repeat are "connected" seamlessly, i don't see problem with this.
There is probably other solutions, but i didn't research much about terrain rendering so i would not know.