Who I have the same questioned this topic

long story short, i want to make sure my texts are being seen. i have a super old phone, and i'm trying to text someone (whom i believe also has verizon).. anyways, in the past when i texted them, there'd be the "black check" in my sent folder. i tried 2 more recently and they've been marked with the "green arrow" now. they havent been responded to so i want to be sure that the person got them... does it sound possible that the other person blocked me (there's the slimmest of chances of this being the case, but i just want to make sure)- i dont get any "message failed" notifications or anything so i don't know. 

on a related note, what exactly do the black check and green arrow mean? some people never get the black check (im assuming because they dont have verizon).  however, some people almost always get the black check, yet on a few instances when i text them, it will never change from the green arrow (i know they received my text since they reply though) - im hoping that's just the case with this person, but i have no idea how to be sure... any help appreciated!