It appears some people are misinterpreting this image.

1: "Designed for solo play" does not mean "you should only solo" or "you can solo everything". If we were to apply this to standard classes the image would say "designed for group play" but we all know this doesn't mean you should only ever play in groups if you're playing a standard class.

The end-game content for blu is The Masked Carnivale, which is solo content. This is the solo play blu is designed for. The end-game content for standard classes are raids, which is the group content they're designed for.

Just because something is designed with a specific task in mind does not mean that's the only thing it can or should do.

2: "Not suited for parties" does not mean "you should only solo" or "you should only play with other blu players". It means that while blu is classed as a dps, it does not fit into the trinity system the standard classes are designed for.

Blu is a jack of all trades but master of none. It has a lot of utility and high damage potential, but past the point of cheesing out 1k needles, a standard dps class does more reliable damage. It has one strong heal but it can't compete with a healer's varied toolkit and it has no res. It can tank but it's not as durable or efficient at aggro building as a real tank.

On top of the above, blu does not learn abilities by lvling up which means the effectiveness of one blu player to another can wildly vary. If you have a lvl 50 standard class in lvl 50 group content, you know that the player has access to the necessary abilities to complete it. Not the case with blu. Having a blu in the party can mean having to carry a dead weight, or having a very useful team mate.