There are two main reason why you would want to craft Blacksmith items:
Change items bought from the Clan Inventory with expensive upkeep costs (usually Legendary items) to more powerful versions that do not have any upkeep costs
Convert Weapons/Warriors that have fallen below the level at which their defence and/or attack are employed, into useful items again.
Convert a used weapon into a more powerful one using easily obtainable parts (coin purchase or gifts) but be careful the craft only uses 1 of the item you are currently using.
But beware there are many items which when crafted actually lowers your stats. if you do not get the timing right. The Poison Stroke is a prime example of an item that needs the correct timing (level wise). Until your minimum attack or defence goes above 49 (equivalent of the crafted Noxious Giant Spider [49/49] or the Asgard drop item Longship of the Gods [49/49]), you will lose out by crafting this item as the att/def of the 2 items used to craft it give you 103/103 as opposed to 57/57 for the crafted item.
Before you consider crafting it can be quite useful to see what the lower limits of attack and defence are for your warriors and weapons. The simplest way to do this is to go to Profile -> Battle Strength then click on Clan Warrior Defense: to display a list of the warriors you are using (and similarly for weapons) for defence and attack. At the bottom of this list will be the lowest item you use. Hovering over this item will pop up a tool tip that gives you the stats for that particular item. If the stats of the item you will be using as ingredients of the craft are lower than the low item stats you will not lose anything by using them to craft a more powerful item.
Blacksmith items have specific unlock criteria closely associated with Lands, Levels and Adventures. Some Elite and Legendary items may appear twice. Once where the initial item becomes available and again when the unlock level is reached.
These land tables may differ from the official ones used in the Blacksmith section of the game. This is because the game lists the craftable item in the land where the parts first become available rather than the level you can craft them at For example the Elite Compass Shield is unlocked at level 20 but its location requirement is Aegir's Sea which begins at level 40 making its effective unlock level, 40. Where this occurs we will try to list the item at its effective unlock rather than the stated one.
Sometimes when you craft an item and realise you have made a mistake by doing so, it may be possible to reverse the craft. This is mostly true of Common-Elite-Legendary items. For some reason as well as the item you want to reverse craft, all reverse craft recipes use 1 scout.