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Instead of directly farming these another great way to get them is by opening the "Moldy Bags" in the game. These have an extremely good chance of giving you a fine crafting material (a blue one, like this one) and are rather cheap on the Trading Post. Usually ranging from 20 - 50c a bag, depending on the type of bag.

While you won't get rich buying bags, opening them and selling the reagents on the Trading Post you definitely will be able to stock up your collectibles tab at an insanely fast rate. I made a video on how this method works, if you'd like to see it: Fine Crafting Material Farming Video for all skill ranges.

Here are a list of all the bags in the game so you know which type you'll need to buy for whatever item you're after.

Tiny Moldy Bags (0 - 50 Components)

Small Moldy Bags (25 - 100 Components)

Light Moldy Bags (75 - 175 Components)

Moldy Sacks (150 - 250 Components)

Medium Moldy Bags (175 - 300 Components)

Large Moldy Bags (300 - 325 Components)

Heavy Moldy Bags (325 - 400 Components)

I find killing trolls in Frostgorge Sound is the fastest way to farm these. Vials Of Powerful Blood Farming Spot

I can not find the skelks level 80, you know where are they at orr?

You can transmute these with 50 vials of potent blood, 1 vials of powerful blood, 5 philosophers stone, 5 piles of crystalline dust... 

how many do you get?

Just did it! Man im pissed.. You will get 5 Powerful bloods and you loose one in the proces so thats like 4 at total in exchange for skillpoint (philosopher cost) and materials Tanex01 listed. Do it if you are filthy rich. Otherwise you will be sad panda like me now :)

The recipe for the Mystic Forge gives between 5-10 powerful blood.

Where i can farm this? ive looked everywhere

Quote from Bocina »

Where i can farm this? ive looked everywhere

It drop well from lvl 80 sharks and skelks.

Basically you kill the same mobs you would at lower levels. Level 70+ bats, trolls, skelks, sharks, and skales. However, I haven't come across any high level bats.

There are level 80 sharks? In Orr?

Yes, and they are in the water

Because land sharks are far more difficult to find.

@step93: the Reef Skelks in Southsun Cove can drop this blood. The Alpine Skelks in Frostgorge Sound (near Sea of Lamentation) have dropped this for me at least twice).

@Jel3us: In the Grawl Fractal of the Mists there are high level bats (in the part where you are following an old mine cart track) that sometimes drop Vial(s) of Powerful Blood (but also drop Vial(s) of Potent Blood).

@Pherixian: at least since they fixed Ancient Shark Pistrix in Sparkfly Fen ;)