"They appear to be tall."
Translation:Zij blijken lang te zijn.
Some verbs followed by an infinitive require "te" to be used, some do not. That seems to be just a matter of learning which is which. However, we also have some requiring "om" with the "te" and I have seen no guidance on that.
As examples from my vocabulary list, vragen, verlangen and streven use the om. Hoeven, blijken and bweren do not.
I'm sorry to disappoint you but a lot of verbs have different prepositions ('voorzetsels' in Dutch). Most verbs don't have any fixed preposition but some do (here is a list if you're looking for a specific verb: http://nl.wiktionary.org/wiki/WikiWoordenboek:Lijst_van_vaste_voorzetsels). This includes 'vragen om/vragen naar/vragen over' etc. (It's more or less similar to English) The word 'te' is used when followed by an infinitive, but only in special cases such as these: http://www.taalblad.be/zinsbouw/te-en-infinitief Of course 'te' can also meen 'too' as in 'it's too far' = 'het is te ver'.
It might seem hard to get to know all this but it's just a matter of practice. Dutch people sometimes make mistakes themselves with the prepositions of the less often used verbs.
Note: the verbs 'kunnen, mogen, moeten, willen, zullen' will never be followed by 'te', but they can be preceded by it if they are the infinitive in one of the special cases (see above)
"Zij blijken om lang te zijn" does not make much sense.
In some sentences 'om' is added like 'It is nice to be tall' = 'Het is leuk om lang te zijn'. In combination with 'appear/blijken' 'om' is not added hence 'Zij blijken lang te zijn'.
Trying to come up with an explanation maybe its the use of of a word like 'leuk' or not.
'They want to be tall' = 'Zij willen lang zijn' 'He hopes to be tall' = 'Hij hoopt lang te zijn'
'They are afraid to be tall' = 'Zij zijn bang om lang te zijn'. 'He wears shoes to be tall' = 'Hij draagt schoenen om lang te zijn'