What warranty does the NSB100FT BLUE+ carry?

The warranty term for the NSB100FT BLUE+ is confirmed in writing with your quote. The figure published on the data sheet is the design life: 12 or more years at 68°F (20°C), with 2050 cycles at 50% depth of discharge. On a cycling site the cycle rating is usually the number that sets replacement timing, so it is the one to compare between suppliers.

What is PSoC cycling, and why does the BLUE+ suit it?

Partial state of charge means the battery spends its working life somewhere between full and empty rather than sitting topped up. Ordinary stationary lead-acid hates that, because sulfation builds on the plates when the battery is rarely brought back to a full charge and the lost capacity does not show up on a float voltage reading. The BLUE+ uses a thin plate pure lead construction and a fast-recharge acceptance that lets it come back up quickly between events, and NorthStar rates it at 2050 cycles to 50% depth of discharge on that basis. In practice: if your site discharges the battery on purpose, on a schedule, this is the line to look at. If it discharges once a year in an outage, you are paying for cycling you will never use.

How do I choose between BLUE+, RED and HT RED?

By duty and by cabinet temperature, not by capacity. BLUE+ is the cycling build with a 12+ year design life, and its warranty term is confirmed with your quote. RED is the float standby build with a 15+ year design life, and EnerSys warrants it for three years. HT RED is the hot-cabinet build, rated for sustained operation at 104°F, and EnerSys warrants it for four years. A cabinet that runs hot shortens the life of a BLUE+ or a RED block, so send how the site uses the battery and how warm the cabinet runs with your quote request, and a specialist will tell you which of the three the install calls for.

Can I add one new block to an aged NSB100FT BLUE+ string?

Rarely a good idea. In a series string the weakest block sets the runtime, so a new block put into an aged string ages down to the string rather than lifting it, and the replacement delivers less than its own sheet says. This bites harder on a cycling application than on a float one, because the mismatch is exercised every cycle rather than once a year. If the string is several years old, the honest answer is usually to plan the whole string and keep the good survivors as spares. Send the install date and the last set of float readings and a specialist will tell you which case you are in before anything is ordered.