The billowy puffy cloud effect we're are used to would not be present because it is the atmosphere pushing back that causes it. In space, each bit of debris would basically keep going in a straight line. Really an orbit around the local gravity well, but it would look like a straight line.

Watching the LEMs take off from the moon looked funny to me as a kid. The initial blast didn't billow. It didn't look like a blast so much as a bunch of sparks. I've been thinking about this get a while.

Actually, there could be a little of billowing, depending on how the ship blows up. If the blast starts with a relatively slow expanding has cloud and then later (possibly just a fraction of a second later) there is a faster expanding cloud, there might be some billowing when the fast has catches up with the slow. That could happen if relatively low energy stuff like the hull, cargo, life support, for example blows up first, and then high energy stuff like fuel and weapons blows up. Besides moving faster, secondary blast would probably be hotter and brighter.