It's most likely recycled HDPE plastic (milk jugs, soda bottles, etc).

Even though you're growing an acid soil crop it's still considered safe. Beyond that, the plant roots do a good job of excluding any potential toxins and you're not eating the roots...or leaves.

Check to see if there is a recycling symbol/number stamped or molded on the container. More than likely it will be #2 HDPE as nc-crn suggests. I have a compost bin, EarthBoxes and a number of other containers with this marking. I would urge you to follow the search below and read up a bit about various plastics and their uses/reuses. Draw your own conclusions as to whether or not it would be safe or right for you.

Here is a link that might be useful: plastic recycling numbers

There is no number stamped anywhere.