I played Morrowind after Oblivion and while I enjoyed exploring the game world, I had little patience for it's dated gameplay and combat mechanics. As a result, even though I had it installed I never actually traveled to Solstheim and played Bloodmoon before I tired of the game and moved on.

I still have the game installed, and I've been tempted to go check out Solstheim, but I can't decide whether doing so would be more likely to enhance or diminish my enjoyment of Dragonborn. On one hand, it may set me up for disappointment if the Dragonborn version of Solstheim doesn't live up to the Bloodmoon version or if so little has changed that it ruins the sense of exploration in playing the new DLC. On the other hand, Dragonborn may be best appreciated by those who played Bloodmoon and can better understand the significance of the changes made over the past couple centuries.

Decisions, decisions ...

It's been 200 years, so you won't meet that many characters from Bloodmoon, the most we can except is probably a cameo of one or two of the centuries old Telvanni and some tombs or books about what happened during Hircines little party. The landscape will look differently because a) there's a lot more Dunmer now and the game looks different in general, so you won't miss anything major if you don't play BM.

However, I can really recommend Bloodmoon. I hated Morrowind & Tribunal for the most part and even I liked the trip to Solstheim. If you can put up with MW once more, just play the Solstheim quest line as a werewolf, the combat doesn't seem as dated since it's claws-only. Install a mod like Morrowind Overhaul and the graphics don't see as hideous, i.e. it has more colors than brown, dark brown and really dark brown.

You will not loose nothing by exploring a little. The island should be very changed in Dragonborn, because of the dunmer occupants and the ashes.

I would go ahead and do Bloodmoon. There are certain things of interest that will get you familiarize with that island. One is the Skaal and the other is Raven Rock (there are others, but these 2 are key from what info we got so far).

*Side note.... I played Morrowind, especially Bloodmoon a month before Skyrim was being sold. It got me in the mood of the climate and got re-accustomed to the Nordic culture. I was glad to do that, hence Bloodmoon can do the same for you until DB is out.

If you don't have the patience to play the game. You can always read up on the story. Either at UESP Wiki or The Imperial Library. I enjoyed Bloodmoon alot, it was a welcome change of scenery. You don't have to beat the Morrowind or Tribunal MQ to enjoy it. Alot of the npc's are unaware at what's been going on in Morrowind. For example: Ask a Nord about Almalexia and they'll respond with a "who?"

There may be a number of throw-backs and easter eggs in Dragonborn, so I would recommend playing for the in-jokes if nothing else.