I got into a disagreement after my opponent declared a blocker -- he argued I couldn't use bloodrush to give my creature +4+4 and Trample after the creature had a blocker declared. However, I've been told that when an opponent does NOT declare a blocker is the intended time to use blood rush to bump up the damage.
Is there some technicalities to the rules that make this a sensitive topic that I should know about before going to my next FNM, or was that merely a misinterpretation of the rules on my opponents part?
That was a misinterpretation. The creature is attacking whether it's blocked or not, so you can Bloodrush either way.
It was a misinterperetation on your opponent's part. Once a creature is an attacking creature, it stays an attacking creature until the end of combat. This includes after attackers but before blockers are declared; after blockers are declared but before damage is assigned; and even, if for some obscure reason you wanted to, after damage has been assigned and applied but before the combat phase ends and the second main phase begins. (This last one is essentially irrelevant for BloodRush, but has its uses for things like Dispense Justice and a few other effects. But that's not what you're asking about.) As long as you have priority and it's after you've declared attackers, you can activate Bloodrush.
Actually, now that I've already posted, there's a minor technical clarification I need to make. Once a creature is an attacker, it's an attacker until the end of combat or until an effect removes it from combat, of which there are a couple (Hollowhenge Spirit comes to mind). That sort of thing must be an effect that explicitly states that it removes someone from combat, however, and simply blocking isn't such a thing.