Taxis in Buenos Aires are Black and Yellow...
If you are on the street then my recommedation always is to grab those who belong to companies...You can recognize Radio taxis because their cars have a visible Company name and telephone number on the passenger doors. If you have time call a cab to your home...(except rainny days...better to catch one on the street)
Their rate during the day time is: pesos7.30 + trip and after 10pm: pesos8.70 + trip
Good luck! Just hold up your hand and a taxi stops!
If you are going to visit BA rent a flat at www.andreasstylebarentals.com