The modifiers apply when using Chain Affinity, since you can use it with 0 TP, you'd get a 1.50 mod anyway. Just having TP and casting a spell does nothing, TP actually has to be consumed with Chain Affinity for it to do anything.
For example, if I'm doing self-light, I'd use savage blade followed by CA Cannonball, the TP return I got from savage blade will go back to 0 since CA will consume that when I cast Cannonball. Without CA, just regular modifiers apply which for Cannonball is Defense, STR: 50% and VIT: 50%. Probably a bad example for modifiers, but explains how TP is used with blue magic.
With Death scissors, basic modifier is STR: 60%. With Chain affinity you then get the TP modifier
0%:1.50 150%:2.75 300%:3.25
Those modifiers only apply with Chain Affinity, and from what I understand, not all BLU spells are effected by TP, even with Chain Affinity. I'm assuming that the TP effects accuracy for the other spells, though it's not stated in the help text but I could be wrong, I'm only saying this out of my experience with BLU.