Hey VexNxll,
This sounds like a very strange issue indeed. Black and purple squares typically indicate that a texture doesn't exist at the path where Minecraft expects it. Wurst doesn't typically conflict with texture packs, so I'm not sure what would be causing this.
If I understand correctly, you have the resource pack installed on a server, so that people need to accept and download it when joining. Did you accept the resource pack when you joined with Wurst installed? Could you give me a copy of the resource pack for testing? Also, which Minecraft version are you using?
Let's get to the bottom of this.