Low maintenance plant to complement Kniphofia?

I'm helping out some friends by adding a bit of colour to their garden and they've already got Kniphofia growing on this border but it needs some buddies as it looks a bit bare on its own.

I've been thinking of something blue or purple to contrast the colours nicely but the only plants I can think of which arent too tall are Rosmarinus, Lavendula and Vinca. I'm not sure that they have the kind of look that I'm going for and I need something which flowers at the same time.

Does anyone have any other suggestions?

Thanks!

 

"Platts Black" Phormium. It's yellow flowers would go well with Red Hot Pokers.

There' a red variety of Phormium too.

 

Herbaceous geranium. ...... Voted plant of the decade at Chelsea this year .....