I'm using a round grey plastic (outlet type floor box) out side for a pool circuit as a pull box because the run is long. The plastic blank cover that that comes with the box.. is it ok to use that as a cap outdoors? No outlet will be there, just a junction box. Or is there a separate W/P cap I should use?

I would have a problem with the use of the materials you indicated you were using for this installation. They do make listed boxes for that application. Hopefully it's not for the pool light.

Ever think of using a handhole box?

They can come in real handy sometimes.

No its for a fountain. Its the same box they use in cement for floor boxes, so it is weathertite. I was just wondering about the plastic cap they give you. It looks to be weathertite too. Unless they make a different W/T cover, I have never seen one. If they do I will go back and change it.

The plastic cap they give you is to keep out concrete and debris when this box is used per its listing. It's not a finish or waterproof cover and the box isn't listed for the use you describe as they can only be finished with floor box covers.

Do they make a blank cover for that box?

Why would they make a blank weatherproof or water tight cover for a box the IS NOT LISTED FOR USE AS YOU HAVE DESCRIBED? I can only expect that you don't plan on having it inspected or you don't care. Change the box and get one with the proper cover.

Why would it not be waterproof, it is PVC just like the pipe going into it. If there is a special cover for it I will get it, or I will change the whole box out. I'll check with my supplier. Don't know of any other better weatherproof box.

Aldav,
As others have said, you NEED a handhole box for this application, a floor box is the wrong gadget for the job. Ask you supplier for a "Christy Box" and see what they have.

Check out their catalog: http://www.christyconcrete.com/elec_com_boxes.html

You probably want an N9 or FL8. Set it in a base of crushed rock so the bottom can drain, bring your pipe in the sides or bottom, and pour your finsihed concrete around the sides. Use waterproof wirenuts if you are splicing in this (though it's better if you don't), and provide bushings on all your conduit.

Quazite makes something similar. You can get boxes from 6"x8" all the way up to "gigantic".
They're at www.quazite.com