These small pores have "banana-shaped" cells called guard cells. When the stomata is open carbon dioxide can enter the leaf, while allow water and oxygen to escape. How many stomata are present on a leaf is called stomata density, and plants can change their gas exchange rate by varying the density in new leaves.
The blue color in this specimen is from Methelyne Blue stain used to highlight the individual cells structure against the background. Specimen staining is just one technique to help students researching microscopic specimens to determine what they are (their morphology).