Yep - good old sensor dust. You're not going to be able to see it with the naked eye - it's ridiculously small. A good loupe might allow you to notice it. But generally, you took the right approach with the blower first - it's always the first go-to method. When the blower doesn't work, you have to move to the next steps.
I consider the next best option after the blower, a step 2 for me, is to use a sensor brush. These static brushes with very fine bristles can often dislodge non-sticky dust that the blower cannot get to. They range from $20 wood handled brushes with fine bristles to $100 electric specialty brushes that spin and charge.
If THAT step doesn't work, then you're on to step 3 and the final step: wet swabbing. Again, wet swab systems can be cheap, from around $20-30 for some plastic swab wands and pec-pads and a small jar of E2 solution...to up to $70-100 for kits with special wands and multiple pads and fluids for different purposes. Wet swabbing will pretty much get to anything the other two methods couldn't - even sticky spots, sap, spit, water spots, etc. Some people are a little scared taking this last step - it isn't hard, but requires patience, steady hand, and some learned technique. If it scares you too much, almost any camera shop can perform sensor cleanings for you for $20-50.
I have a $40 blower, a $20 brush, and a $20 wet swab kit - I bought them all 3 years ago with my first DSLR, and have used them all extensively over the past 3 years. Certainly the $80 spent has worked out far better than $40 cleanings for the past 3 years, so I'm glad I went this route (those of us who do a lot of outdoor wildlife/biring/nature photography encounter far more dust and dirt and ugly conditions, with in the field lens changes, than most people, so for me I pretty much use a blower weekly, a brush ever 3-4 weeks, and wet swab twice a year).
