From the bloggers at Do It Yourself RV.
A blue boy is another name for a portable waste water container. These tanks come in a variety of sizes, from around five gallons to upwards of 30 gallons or more.
While they’re sometimes made of blue plastic – hence the ‘blue boy’ name – they’re used to transport ‘greywater’. Gray water (or graywater) is the name for waste water after it has passed down your sink or shower – but not from the toilet.
If the water comes from the toilet, it’s called ‘black water’. Some RV parks prohibit blue boys from carrying black water, but yes you can transport poop (blackwater) if need be.
To read the full story by Do It Yourself RV, click here.