The blend feature in RoasterThing allows defining blends and then making up the blend to roast. It properly deducts from inventory and adds to the blend. At this point it only supports pre-roast blending.

First you need to define the blend using the Blend Designer which you will find under File=>New Blend on the Coffee Inventory List. Blends may consist of up to 6 different coffees in any percentage mixture you like.
If you check Hide in List then this blend will only be visible when you pick View=>Show Only Blends. The right most column shows the amount of each coffee in stock so you'll know if you have enough to roast this blend.

Then you use the Blend Builder located at File->Build Blend on the blend detail page. This will show you how much of each coffee to measure for the target roast and will deduct those amounts and add them to the blend. They you roast the blend just like any other coffee.
The columns on the left are how much coffee to pull for each different coffee. The second column will show how much for each batch if you're roasting more than 1 batch and want to split them evenly. The "% Remaining" number allows you to pull enough coffee to make the amount you wish to sell after accounting for the the weight loss during roasting. That's why in the following 2 images the total at the bottom of the column is larger than the requested weight. If you want to think about pre-roast weight, then set "% Remaining" to 100%.