As you may tell from the title Blue Dye is just really blue dye in Runescape.. It can change the color of any regular cape to blue (as the red and yellow dye also can). If you mix it together with red dye you make purple dye. If you mix it with yellow dye you make green dye and so on.
Blue dye is also in fact a good way to make some money, doesn't give a lot, but it doesn't take to long to make.
Step 1: Where to Obtain Your Ingredrients.
To make Blue Dye you need 5 coins (5gp) and 2 woad leaves which is bought buy Wyson the Gardener located on the map below. talk to him and click continue. Then click "Yes please, I need woad leaves." From then on just keep clicking continue. You don't even have to move your curser.
(When you get to the part when Wyson asks you how much you will pay him, pay him 20 coins for he give you 2 woad leaves for being generous. 5 coins and 10 coins won't work and 15 coins only gets you one woad leaf)
Buy as much as you'd like depening on how much money you have. 10 woad leaves are 100 coins. 20 woad leaves cost 200 coins and so on.
Wyson the Gardener is located in the Falador Park, east of the small pond near the little building.
Step 2: Making Blue Dye.
After you obtained the amount of woad leaves you wanted, it's time to make the dye.
Making the dye is in a different location so buying many woad leaves are advisable.
In the town Draynor Village there is a witch located north of the library. The entrance to her house is inside the circle.
Right click on her and click on "Make-dyes". Three choices will show and you can choose which dye you wish to make, each require different ingredients. Click on the option in the middle and she will make your Blue Dye. Repeat this process until all your woad leaves and coins run out. (5 coins per dye). She will even do a weird emote near the cauldron.
If you wonder what the witch's name is its Aggie.
(30 woad leaves and 75 coins gives you 15 Blue Dye).
Step 3: Selling Your Blue Dye
The best place to sell your Blue Dye is in the G.E. located north of the large Varrock Bank near Barbarian Village And Cooking Guild.
Blue Dye sells for 314 to 348 coins each, being 331 coins the mediam price.
(TIP: These prices change over time)
(30 blue dye gives you 9930 coins as mediam price. Low - 9420, High - 10,440)
You can use blue dye if you want to make blue capes. (You can get free capes killing bandits on the way from Draynor Village to Falador and vise versa)
Takes a while to sell, but can be made in 30 minutes (30 blue dye)
Note: Only members can be able to use all Grande Exchange Boxes, non members can only use two of the sixe boxes avalible!