I had one of my worst plumbing related disasters because of those damned blue things. I think I may have told this one before but it's worth a re-tell given the topic.
I was at a customer's house to replace the bottom entry inlet valve on the old lady's toilet. I took my wetvac into the bathroom to empty the cistern and noticed there was a blue goo in there but knew my wetvac could easily clean it out so ahead I went. It took only 30 seconds to vac out the cistern and all was going well until I turned around to pick up my spanner. I was greeted with the horrific sight of everything behind me sprayed blue with that dye. It was up the vanity unit, on the wall, all over the thankfully dark blue carpet (thank heavens for small mercies) and it looked bad. The blue block had created a foam inside my vac and blown it out the exhaust! I nearly had a panic attack but got on with the cleanup using the wonderful wonderwipes. I was amazed at how well I managed to clean the room up until I made my second big mistake with the contents of the wetvac - I poured the contents into the shower tray and rinsed it down.
It dyed the white grout blue in the shower tray. I started having a nervous breakdown and didn't know what to do and considered resigning. The I had another bright idea and took a sponge, and sponged the blue stuff over the whole enclosure which made the grout at least match. It actually looked quite good! The homeowner was a very elderly lady in a huge house with five bathrooms and 2 carers looking after her. I confessed to the carer what I'd done and she took a look and said it looked fine so not to worry! I was so relieved I took the rest of the afternoon off and went to the pub.
Don't wet vac that stuff out of a cistern unless you can control where the exhaust is going.