Although this is sad news, it must be noted that you can learn allot from this experience.
1. Identification of a problem before it becomes life threatening for the fish
2. Prevention is better than cure, buy this I mean that its always going to be best to quarantine your livestock before adding them into the community as the fish which go through the LFS channels will always arrive with issues or develop issues while at the stores holding tanks. The Blue tang is infact a very hardy fish and would have behaved fairly normally even with the flukes present, right up until the last.
I would guess that the fish was severely infected with flukes from the gills to skin and lastly the eyes. Regular skin flukes are common on fish all the time and they are able to tolerate this for extended periods. This is different to something like brooknella which degrades the fish quickly.