Strange leaves

[Irish Miss]

I purchased a blue hydrangea from my local market and it was just beautiful.
When the flowers faded, I snipped them at the first junction.
I placed the plant outside on my deck when the weather warmed.
My daughter had told me to place flathead thumbtacks in the stems that I had trimmed to prevent insects from getting in and killing the plant.
I did this, but the leaf edges turned black. I have removed the black leaves and there is new growth at the base of the plant.
It is still in the container. Can you tell me what is happening?
The leaves are not wilted and appear healthy, but the edge is all black.
Any help would be appreciated.
Thank you.
Irish Miss

[vintagejuls]

I've never heard of putting thumbtacks in a plant. Maybe that is the problem since it doesn't seem to be natural.