I found the solution - and here it is, for anyone who may also encounter it.
The pulse range for the Talon 35 (*) is 1070us low to 1940us high. The Spirit uses 1100us to 1900us by default (*) which is just enough for the ESC to not arm (neither in low for flight nor in high for endpoint configuration).
(*) I don't know if these values are typical for the Talon 35 or if they are specifically preset in the Blade 360 for the AR7200BX. Anyhow, they are just slightly outside the default Spirit range and enough for the ESC to not arm no matter how much travel, sub trim, and whatnot you change for the throttle channel in your transmitter.
So if you adjust them in the Spirit configuration software, you do not need any trimming in the transmitter.
Cheers,
Raoul