Most fans on a PC will accumulate amazing amounts of talc-fine dust and a dust laden fan will not do its job.
Just blowing at it may not be enough. The stuff cakes - over time........What you do about it depends on your confidence/daring.
A soft watercolour brush can work wonders with a puff or two from your lungs, but first you need to isolate the fan as far as possible, avoiding electrocution or static damage, and also do whatever you can to collect the dislodged dust.
I have a gravity feed anthracite boiler (ancient stuff, and Yes I don't still use an abacus). The boiler suits my needs but started to go out on me in spite of flue cleaning and the rest.
Its fan was loaded with fine ash dust. This was brushed clean and the result was like new, back to long periods of idleness and a brisk burn when the thermostat cut in.