Could sympatric speciation occur in humans, for instance, separating blue collar workers from white collar workers?
No, it won't happen
Even geographical isolation failed to allow speciation in humans (see Does it make sense to classify all humans in a single species?). So if there is some small reproductive isolation between working groups, it is definitely not going to be strong enough to cause speciation.
In an imaginary world
If there was strong reproductive isolation
In an imaginary world where there is a more important reproductive isolation that lasts for long enough between blue and white collar workers, sympatric speciation might be possible. Such reproductive isolation could be caused by very important social stigmas.
Wallace effect is a type of speciation that is mediated by a reproductive isolation that has been selected for as fitness of offspring is higher when mating is assortative.
There is probably relatively little genetic predisposition to being blue collar or white collar worker. But in an imaginary world where there is very strong genetic predisposition to being a good blue collar or white collar worker, and where worker quality will affect income which will affect fitness, then yes, wallace effect could be a thing.
But really, we do not live in one of this imaginary world. In our world, speciation won't occur between blue collar and white collar workers.