Black spots on my flags

[farida]

Hi all
Found this forum on a google search & hoping that someone can give me some advice.
After being down for about a year,we recently had our Indian stone flags cleaned - just power washed with water. They've come up really well....apart from as you get round into the back garden, the cleaning has revealled flags with black spots on them. I can only describe them as looking like ink spots(you know, with a 'spreading' edge) and they get more and more frequent as you get closer to the area of the garden that is in the shade of a couple of very large trees.
I'm wondering - a)does anyone know what it is; b)can it be cleaned and c)is it likely to have anything to do with the trees behind the house?
I await any reply with anticipation and thanks in advance!
Farida

[lutonlagerlout]

sounds like algae
read this page
cheers LLL

[farida]

.....looks like I'm looking for the best deals on bleach at the supermarket this weekend!
Thanks, I'll give it a go & let you know how I get on
Farida