Thank you for the post and hint to the BLUE card.
The BLUE card was/is also offered in Germany. Many moons ago itīs fresh design in comparison to the classic design of the green card was used as a unique selling argument to younger people. @:-)
It attracted many new customers and also entrepreneurial people which thought that the classic design of the Green card was not stylish enough.
When introduced to the market some (nice female) colleagues of me wondered about how they have put the blue hologram into that card. They havenīt seen a credit card like that in the German market.
You may have a look here:
https://www.americanexpress.com/de/content/blue-card/
This card is fully functionable and has no downsides. In my experience the spending limit is calculated like for the other cards also. The only disadvantage of the German version is that they charge an amount for the participation in Membership rewards. So double check if itīs really free also in the second year.
Therefore there is no reason for not grabbing the spending bonus and open this card as a second Amex card.
The reason why you may not see this card too often is that they start sending the cardholder upselling letters to the next higher colours. They are not aggressive or annoying in doing so, just send the invitations by mail "Danke für Ihr Vertrauen...... " and offer the higher colours.
And then many cardholders start moving up to green, gold or even Platinum.
So I am not able to find any argument against the Amex BLUE card.