Legarrette Blount is not and will never be an every-down back - and that's okay. The first part of that sentence should be obvious: Blount is limited to running between the tackles and doesn't have a lot of quickness to him - he can't make someone miss in space other than by jumping over them or powering through them. Besides that he can't stay on the field on passing downs, ostensibly because of struggles picking up the playbook, and he has a fumbling issues with 9 fumbles in his two years in the league.
All of those are legitimate concerns. But none of them matter. Well, except for the fumbling. That's a real problem. But the fact that he isn't on the field on passing downs, that he can't run outside, that he isn't shifty? It doesn't change anything about his role: to be a between-the-tackles runner. That's what he's good at, and you can build a running game around that. You just need complementary players to create the illusion of an every-down back.
Having different running backs for different roles is not a problem. In fact, almost all NFL teams go about their business that way. They have a back who primarily runs the ball, and when they go to pass-heavy personnel their third-down back comes onto the field.
The best example of this practice can be found in the form of the New Orleans Saints. For years they've shuffled running backs in and out of the lineup for different roles. Reggie Bush was drafted to be an every-down back, but turned into essentially a glorified receiver in New Orleans. Pierre Thomas runs between the tackles, but was also their designated screen back - until Darren Sproles usurped that role. Sproles is another complementary piece who will never be an every down back, but who can be used in specific ways to maximize his potential. And then the Saints have two more players who can function as the foundation of a running game: Mark Ingram and Chris Ivory.
Over the years the Saints have found different ways to use their backs and use their entire stable of backs in a variety of ways. That's also the way the Bucs can go and have gone in the past: they've had a stable of running backs with a variety of different roles. The good thing about that is that you're not dependent on one player, if you do this correctly and build in some depth: if one player goes down, another one can take over his limited roles fairly quickly.
This is part of the reason why I don't think drafting someone like Trent Richardson high in the draft is any kind of answer for the Tampa Bay Buccaneers. It's fairly easy to construct a competent stable of running backs with different roles without having to spend a high draft pick on a running back when you could easily spend that pick elsewhere - where you can't use a variety of role players to fill a need.