Is the choke staying closed perhaps?

On my 25 year old Honda, the spark plug has always been black.
A puff of blue start up smoke is typical if the oil level is even the slightest amount over the full line or the mower is stored on a slope.

Constant black smoke is NOT normal.

is this equipped with the auto choke system?

does this have the lever that manually engages the choke?

does this have the system in which the throttle engages the choke?

I guess if you are in Australia it just was spring.
Anyway, maybe you had it on its side and oil is on the air filter.
Yes, give us the model number.
And Blue smoke is oil. If it sat a while it could be the problem.

Its running 'rich' (too much fuel being delivered to the combustion chamber) which is very bad for the engine. It dilutes the oil on the cylinder wall causing excessive wear. The float in the carburetor is probably stuck. Pick up the carb side of the mower and drop it from about 6" and see if it clears up. Use some kind of fuel system treatment like 'Seafoam' or 'Marvel Mystery Oil' to clean the system and keep the float free.
Otherwise there may be a problem with the choking system, leaving the choke on all the time.

I

Would a rusty muffler inside cause it to throw black smoke if it is stored for mths subjected to humidity but not in the rain....