What do you think about truecaller apk app?

I use True caller also, downloaded from Google Playstore. So far works great in filtering out unknown numbers.

What if its an emergency call coming in?

Blocking shouldn't affect emergency calls. You can customize and whitelist numbers in the app settings.

when i was scrolling down /k i swear i read the title as block spem cells..

Goes to show wads always inyour mind. Lol.

I'm using Truecaller.
It can block the calls that you marked as spam.
At the same time you might expose your contact names and numbers to their database.