I have a feeling your code is operate so fast that you can't see it. Try commenting out the microseconds and bringing delay up to a second to see if it starts to blink with more repetition.
IR diodes produce absolutely no visible light - but digital cameras (unless they have a really good IR filter) can usually see them - phone cameras are most likely to lack an IR filter.
The IRF510 is not a logic level MOSFET and won't work properly from 5V gate drive. You must use a logic-level MOSFET or level-shift the pin's output to 10 to 12V.
No so - it is possible sometimes to see a faint glow from bare (without a proper filter) IR illuminators, though I agree, digital cameras are usually a good bet, and 940nm is quite a long wavelength
Standard IR diodes are ( or used to be ) narrowband and dont spill into visible. Perhaps newer high efficiency ones use quantum wells and produce a broader spectrum? IR remote control wants narrowband emitters and receivers to help filter out ambient lighting which interferes with wanted signal - IR illuminators just need high efficiency.