1. They convince you to hand over your Blockchain wallet's login credentials. With that information, they access your account and click on the settings tab:
2. Within the settings tab, they click on addresses:
followed by + import bitcoin address:
3. A pop up box appears, prompting for a public address or a private key:
In this example, let's use the dormant address 14WWrJMGPo43mAoLXoH53pREAEUAFFW66T
After pasting this address in the box and clicking on the light blue import button, you are prompted to confirm the import process:
Notice the extremely believable balance:
If you verify the balance, it does exist:
https://blockchain.info/address/14WWrJMGPo43mAoLXoH53pREAEUAFFW66T
A scammer who, for example, offers Skype video proof can use this as evidence of having bitcoins. If you are not aware of the watchlist feature, you might be convinced and if you are one of the gullible ones, might even proceed with paying first, thinking along the lines of Hey this guy has millions so he can't be a scammer?! Let me quickly send him that 1 BTC!
Do not fall for this scam. Ask for signed proof of bitcoin ownership.
OP, don't be scammed. Think! Why would they put money on your account and risk losing it, given it's YOUR account and not theirs? Go ahead and follow the above steps. Stopping scams is most effective when you understand how these thieves do it.
Source: https://bitcointalksearch.org/topic/btc-question-2948841 Read this whole thread for more info. Shoutout to warningsigns.