I've been researching BlockCard and I'll put this publicly available information out here for those who want to do their due diligence. Offering >6% cashback on a card is a "too good to be true" scheme. The company has to profit more than that to pass on the >6% rewards. For someone to get 6% rewards, someone else has to lose >6%.

For a blockcard transaction to take place, TERN has to be sold to convert it to USD to make the purchase. So there has to be a seller of TERN (the blockcard user) and a buyer of TERN who is willing to pay USD from its deposits (Ternio). This is how this whole play seems to take place, you can trace all the transactions in each wallet's trades and offers:


AZUF https://stellar.expert/explorer/public/account/GBXJXJGMEXLWNMXR3QIV7HLQHW7C5GANYYCZ3YEHMK4ZGZUPI22SAZUF = USDD issuing authority
This is the USDD issuing authority controlled by Ternio. Has transferred $6,000,000 USDD to N7H2 over time.


N7H2 https://stellar.expert/explorer/public/account/GARBGDV6QQW3GNRQUM4RBI6YLAJYV45QUDKHOCNLTZD4XW5LEPRON7H2 = Ternio’s functional USDD wallet
This holds USDD to buy TERN from IVDX each time a blockcard debit card is used thus essentially paying the $ for debit card transaction to go through. Also buys TERN from VSAA


IVDX https://stellar.expert/explorer/public/account/GCAVYLHJEFLCMGYTB524LUWGSLLVNIZ6CSUTZ6UXJZCQ3JTRM5EOIVDX = Blockcard users' collective TERN wallet
This is the collective TERN wallet for all blockcard users which holds TERN. It sells TERN at the time of a blockcard debit card transaction to N7H2 or VSAA to convert to USDD.


VSAA https://stellar.expert/explorer/public/account/GBMSJSF6EWZCRYRTGMD654WPRG53PDU4CWBRU35KUYUSCBZNT6XXVSAA = Mediator which keeps profiting (this is the key to why blockcard is sustainable for Ternio)
This keeps making transactions in cycles essentially profiting by arbitrage in a captive market controlled solely by Ternio
First: Sells its TERN to N7H2 at higher price
Then: Immediately buys back TERN from IVDX at lower price, thus fleecing the users.
The profit margin for VSAA is usually 14.3% (buys at 0.007, sells at 0.008) or 16.6% (buys at 0.006, sells at 0.007) usually. Thus users collective lose this % to Ternio as they make transactions. While an individual may get 6% profit, looking at the entire user base collectively shows that they will lose money to Ternio. Since USDD is solely controlled by Ternio, it is safe to assume these profits belong to Ternio as the any wallets with authorized USDD trustlines can only belong to Ternio.



One may argue that the profits from this arbitrage is coming at the cost of N7H2 which is Ternio's USDD wallet, so it is Ternio which is losing money. Keep in mind that N7H2 is a representation of users' deposits. Ternio has to return this USDD deposit when users make transactions using their blockcards. In a way N7H2 is the assets Ternio holds to balance its liabilities held in the IVDX wallet. Without forced arbitrage these would be equal. The only reason Ternio can make profits is because it has a captive USDD:TERN market where only it can operate and is free to conduct arbitrage without competition at the collective cost of its users.