Blackspot would not actually be in or on the greenhouse itself unless the plants you have in there have blackspot. Every year it pays to give your greenhouse a good spring clean,say March/April time. I have always fumigated it with a 'smoke' bomb.You can buy these from a garden centre. This gets rid of all dorment and active bugs.You must close the greenhouse door and not enter until the next day.If you can remove plants. Following this I used to spray 'Jay's Fluid' on all areas,floor,glass & benches. This sterilizes the areas.Then finally clean the glass to let as much natural light to you plants as possible. Sounds a lot of work but it is not and well worth it. Regarding any plants in the garden having Blackspot, firstly you need to remove any infected leaves and either burn them or remove from garden area. Do not put in compost bin. You then need to buy a treatment from the garden centre,there are many to choose from.Then treat the infected plants following instruction on the treatment bottle. If the problem persists into next year(after treament) sadly you will be better off removing the whole plant. Hopefully this is of some help.