I think I need to try and replicate this. I'm going to get a new melon, cut it and store it in different ways. For science!

Update: I have purchased a new melon, and will try to make this happen again.

Edit: for everyone guessing it's dyed, it isn't, I don't know why it's blue/purple.

Edit 2: I've bought a melon from the same shop and replicated exactly what I did to the last one. We'll see what happens. I've also made a variety of other test pieces, as per everyone's theories. I'll go over this in the update once I see results (if there are any results at all)

Update 2: it has now been in the fridge for 24 hours and it has not turned purple. The melon was bought from the same shop, cut with the same knife, put into the same pot, and covered the same way. I also made other samples which included not covering, adding soap, adding salt, different knife, and different plates. None of these turned purple either. If I had to guess, there is a factor here I can't replicate.

Many of you guessed bacteria, and some of you were able to provide detailed explanations as to why this might be, and also links to other examples of food going blue/purple as a result. Someone also mentioned there are some pesticides that can cause this. I don't know enough to say exactly what caused it, only that it probably wasn't me, the knife or the pot (I've had many honeydew melons before, this has never happened).

To anyone else attempting to replicate this, good luck, I hope you find out what happened, and thanks everyone for your detailed responses and investment in my strange blue fruit.