I have seen in many puja and rituals sesame seeds being used with milk. Scriptures also mention its usage in certain rituals. So what's the importance of sesame seeds? Why they are used?
To add to the previous answers, here is another interpretation:
"Q: What is the signifiance of Mahalaya Amavasya?
Sri Sri Ravi Shankar: Actually this Amavasya is dedicated to the departed souls. When you leave this body you are guided into another world by a set of Devas or Angels. Pururava, Vishvedeva - these are their names. They come and guide you from one level to another level. Mahalaya Amavasya is the day when you remember all the departed souls and thank them and wish peace for them.
There is an ancient tradition in which the family members take a few sesame seeds and little bit of rice, and then they think of their ancestors and say, ‘May you be contented, may you be contented, may you be contented.’ They say this three times and then they drop the little grains of sesame seeds with some water.
The significance of this ritual is to tell the departed that – If you still have some desires in your mind, know that they are like sesame seeds. They are not significant, just drop them. We will take care of them for you. You be free, happy and contented! There is huge universe in front of you. The universe is infinite, so look forward and go; drop whatever is pulling you back. This is called Tarpana.
Tarpana means bringing satisfaction and fulfillment to the departed. It is done to tell them to be contented and move further. Water is the symbol of love. To give anyone water means giving love. In Sanskrit, Ap means water and it also means love. And in Sanskrit someone who is very dear is called Apta. So, in their memory, you give them water as a symbol of love and life and that is why this is called Mahalaya Amavasya. On this day think of all your ancestors.
In Vedic tradition, three generations on the mother's side and father's side are remembered, and all others friends, relatives, and anyone who has crossed over to the other side. Think of them and tell them to be satisfied. Usually in their memory, people also do some charity, by giving food to some people and to animals..."
Black sesame seeds are used during pind daan because it increases the chance of crow pecking the rice ball (white in colour) by 50%. Pecking of rice balls (pind) by crow is considered essential so that we can be sure of presence of subtle energies (our revered pitras) around us. Crows are usually considered to be vigilant of these subtle energies which our bodies are unlikely to feel.