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Blocktrades is an exchange service.
If I want to convert my steem into SBD, for example, I choose 'Buy' next to my steem balance in my wallet tab.
This will open a new tab, where I can set myself as the recipient, and click 'Get deposit address', which will generate a memo to let blocktrades identify the intended recipient (myself).
If I convince you to send Steem to @blocktrades, using that memo, the resulting SBD will be delivered to me.

Thank you for explaining :) That is exactly what happens.

@blocktrades does not need to scam. It is users using the service who do.

Nice catch dude.

Why a user who is using @blocktrades service want to scam money for blocktrades? why not for himself or herself?

I guess I'm just dense. Still do not grok. I get that someone could generate an address that would send Steem or SBD to @x but do not understand how @x could send a memo that appeared to come from @blocktrades

I would start by sending an insignificant amount to you with a memo that designates me as the recipient of a different, new transaction, via blocktrades, so it looks like blocktrades is initiating a conversation with you.
It says 'blocktrades' in your wallet because that's the exchange service which delivered my bait transaction, with the memo hook.