In summary it takes a total of 5 commons of a particular equipment (by name and tier) to progress from common to mythic rarity.
Equipment card border color indicates rarity: Common is green, Rare is blue, Epic is purple, Legendary is orange, Mythic is red.
Blending follows this pattern:
2 Commons produce 1 Rare
1 Rare + 1 Common produces 1 Epic
1 Epic + 1 Common produces 1 Legendary
1 Legendary + 1 Common produces 1 Mythic.
All of the blends are on Nefty as either regular or super blends This utility will check to see if you have NFTs to blend for most of the blends but not all. You do not log in to this, it just looks up the wallet you put in.