Infestation on Dahlia's

Hello,

I have just been outside this morning and spotted an infestation on one of my Dahlia plants! I noticed the same little bugs last year although it was much later in the season.
They only seem to go for the bedding Dahlia's in the front border of my house. They have literally smothered this particular Dahlia and are starting to work onto some of the others.
I have sprayed them wit a solution of soapy water as they look like black aphids but I was wondering if anyone knew what they were and the best way to deal with them.

Thanks in advance