In the raiding community, ie anyone who is decent enough to care, the word viable means it performs at the highest level or close enough so as not to matter. In DPS terms that would mean the ability between specs to do roughly equal damage or bring utility in place of a small dps loss. In the case of DW blood tanking you get no utility and a serious drop in dps due to the structure of the blood spec. Therefore, regardless of your ability to do it and kill bosses I is NOT viable. In the world of mmorpg suboptimal = not viable.
Just because I can come as blood dps to a raid and kill bosses doesn't mean I should, dps should obviously be frost. "offtank" as dps died in vanilla with 40m raiding. Trying to argue a less than ideal play style is viable is stupid and is why good players laugh at noobs. A DW blood DK app would get denied by any decent guild straight out of hand, again, not viable.