My HUD no longer displays my total Eridium whenever I pick some up. I can't seem to find any total in my inventory, either. Am I missing something, or is visiting the Black Market in Sanctuary the only way I can see my current Eridium total?
The Eridium count is in your inventory, below your current equipment, to the right of your cash total. It's listed in purple.
In this guide you can see it in the first screenshot, with the annotation "12".
I've got an Xbox and no way to capture a screenshot myself, or I'd do it in-game for you...
You've probably maxed out your Eridium count; once you have 99 you'll no longer collect any and you won't see the counter go up. I believe you'll still see the counter in the inventory menu at any time, along with your cash total. If it's not there, it's in the Badass rank menu (I know the Seraph Crystals count is in there).