I did quite a lot of farming for blacksteel gears, and I recorded the statistics of some of the runs. Here they are, in case someone might find it useful:
Where: Land-End ground floor (Act 5)
Runs: 100 runs (restarted the game after every 10 runs)
Fire giant kills: around 20 per run, or around 2k in total
Blacksteel gears received: 29 items: 7 spears, 7 swords, 4 shields, 2 head, 3 chest, 4 arms, 2 legs. The differences between the number of items for different slots are probably due to the small sample size though, and it's more reasonable to assume the same drop rate for each of them. Thus:
Average drop: around 1 drop per 3-4 runs, or around 1 drop per 60-70 fire giant kills
Average drop for a specific item (e.g. spear): around 1 spear per 20-30 runs, or around 1 spear per 400-500 fire giant kills