I have vaped other juices with my Blu and while it works I get a better experience all around from my 510 with the very same juices.
that being said...
Take the blu carto and with a pair of tweezers pull out the insides. ( you can also push out the insides with a unfolded paper clip ) You will find a small ( and i mean small ) plastic "cup that has foil remnants and some filling in it.
Pull the filling out and either discard or if you wish to reuse it, place it on a paper towel. ( I did several carts at a time.
fold paper towel over the filling and squezze out juice.
Rinse filling completely in the sink. Then squeeze out water with peper towel and let sit till dry.
While thats drying, take some tweezers and slowly pull off the foil remnants ( if you can find a better way use that. This method is painstakingly boring)
Rinse "cup" let dry.
Once everything is dry. you can place about 3-5 drops of your new liquid into the "cup" Then roll the filling between you fingers until it will fit back into the cup. Put the filling in and add more drops of liquid until full.
Replace the cup into the cart. Use the tweezers to push all the way down.
You are now ready to vape the Blu again.
If you need to get more filler. Fluval can be found at a pet shop REALLY REALLY cheap. I bought a HUGE bag of it for 2 dollars and I dont think ill ever need to buy more, though I believe after reading here, that fluval is not the "best" filler.
hope that helps
I strongly reccommend however upgrading to a 510 or an eGo as the Blu just doesnt produce like they do. I honestly liked the Blu at first but over time ( two weeks )it just couldnt meet my needs. I have yet to have an issue with the 510 and everyone here is swearing by their ego.