What is that black thing?

I was taking some photos for a project for my botany class and noticed these black structures on the cell wall. I don't know if I'm overthinking this and it's just the cell wall seen at an angle or if they are sclereids or something else. I'm not entirely sure if they are sclereids as I found another section with structures that look much more like sclereids (this is the last picture) and are different to the black structures. Does anyone know what this is?

These are pictures of stem transverse sections of Plectranthus verticillatus