When you squeeze a blackhead, you are essentially extracting a mixture of dead skin cells, excess oil (sebum), and bacteria that clogs the pores. The contents of a blackhead may vary depending on how long it has been present and how deep in the pore it is. The material extracted can appear yellowish or black due to oxidation of the trapped debris. It’s important to note that squeezing blackheads can potentially damage the skin and lead to infection, so it’s generally recommended to seek professional advice for proper extraction methods.