Hi @dalton,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Yes, you can use Dunlop Fibre Cement Jointing Compound to fill out your blueboard before tiling. Just make sure you are fairly smooth with your application, as it is not easily sandable, although this likely doesn't matter as you are tiling over the top.
Let me know if you have any further questions.
Jacob
