I think it is some impurities that was in the bottom of the vat from what they were loading the bottles or jugs with.
Think of them using a 5,000 gallon vat filled with "pure" Castor oil and they run pipes from the vat to the production line to fill the bottles with.
Your particular bottle happened to be at the end of the production line and was filled with the last dregs of the vat as it was empty or close to it.

Of course it could be the very first bottle in the production line to have been filled and there may have been some residual impurities in the pipes to the nozzle that squirts the oil into the bottle. Maybe some machine oil in the pipes.

I would simply gently pour out the oil you need so that you don't disturb the gunk too much and when you get to the bottom you can decide if you want to use it or not. You could also shake up the bottle good and just use it as is. it likely will not hurt anything.