What are these blue pools?

[Bluepinixy]

I sometimes see Sorcerers leave behind these blue pools and I'm wondering what they are?

[Valsacar]

Ice

[Shiny Celebi]

It a upgrade of skill Frost Bolt. It make frozen enemy have 20% chance turn into ice in ground when they get killed (by normal attack/ skill). Other enemies walk over it will get frezze! Of course not working with some enemies who can't be frozen like bosses
I don't really like this upgrade since the chance is too low but my friend suggest using it. I see it sometimes useful in mine/ tombs