I don't think the coaches can be completely blamed (although there's been some shocking coaching appointments over the years, including Phil Davies & Baber+Burnell).
Dai Young had an impressive squad at his disposal, with quality throughout with Jenkins, Felise, ***o, Bradley Davies, Nugget, Rush, Nicky Robinson, Roberts, Shanks, Tom James, Blair, Halfpenny etc. all in their primes/playing the best rugby of their careers. There's still some quality in the Blues ranks, but there's too many positions where the Blues are relying on ageing players (their first choice props are what 37 and 40 years old? and their only genuine option at 8 is a player who's struggling to stay fit), in other key positions they're struggling for quality (lock).
The revolving door of coaches, both head coach and backroom staff has done them no favours either. I rate Danny Wilson very highly, and on the whole he's getting the Blues going in the right direction; but they will always struggle against the best with a shallow squad depth and no investment in players. Peter Thomas may be unwilling to plough more money into the region, but by doing nothing I fear it could simply erode further.
If Wilson can keep his best players fit, the Blues will be competitive to a point, but they will lose close matches when they're forced to bring young rookies off the bench at prop in the last quarter for example.