Why are there five rings in Olympic logo? And what does they symbolize?
The five interlocking rings of the Olympic logo represent the five inhabited continents of the world, namely Africa, the Americas, Asia, Europe, and Oceania. The colors of the rings—blue, yellow, black, green and red—were chosen because at least one of the colors appears on the flag of every nation in the world. The rings are linked together to symbolize the unity and friendship among nations that are fostered by the Olympic Movement.
What countries have black in their flags?
Using the Wikipedia page on National colours, here are countries with the color black on their flag: 
- In Africa: Egypt, Angola, Botswana, Eswatini, Ghana, Guinea-Bissau, Kenya, Lesotho, Libya, Malawi, Mozambique, São Tomé and Príncipe, South Africa, South Sudan, Sudan, Tanzania, Uganda, Zambia, and Zimbabwe	
- In North America: Trinidad and Tobago, Saint Lucia, Saint Kitts and Nevis, Jamaica, Dominica, Barbados, Antigua and Barbuda, and Bahamas	
- Asia: Afghanistan, Brunei, Iraq, Jordan, Republic of Korea, Kuwait, Syria, Timor-Leste, United Arab Emirates, and Yemen
- In Europe: Albania, Belgium, Estonia, and Germany
- In Oceania: Papua New Guinea, and Vanuatu
- Other: Western Sahara, Palestine, and Donetsk People's Republic
In addition, several former countries also have black on their flags.