"The journey is very popular among tourists."
Translation:Rejsen er meget populær blandt turister.
They mean the same, but 'iblandt' can be used both as a preposition and as an adverb, while 'blandt' only can be used as a preposition.
So in the sentence above you can use both, but in this sentence you can use only iblandt:
'Der var mange populære rejser iblandt' = there were many popular jouneys among (them)
The same goes for some other prepositions in Danish: imod/mod =against; igennem/gennem = through; imellem/mellem = between etc.
Actually, as I come to think about it is the same situation with between/in-between in English
That should definitely be "blandt". I can't give you the exact explanation but I feel like "iblandt" is only used for A actually being among B, so you could say, for example, "der er mange kinesere iblandt turisterne" or "iblandt de unge var der også et par gamle". But this is a case of A (a travel, an abstract thing) being popular among B (tourists, a concrete thing). That doesn't work with iblandt.
Tur may be used about any movement from point A to point B, say, a trip to the drugstore or the park or even to the toilet. Rejse is a journey of some length and duration, including multiple forms of transportation, such as a trip to Rome or to the moon or to the other end of the country. You can also use tur for those though, if you want to make it sound a little less formal and daunting and exciting.
This started me thinking about the difference, and in the English equivalents, i.e. tour and journey, tour would be used if you are viewing things along the way, and journey if you are travelling between two points (to then maybe see something when you arrive at your destination. Don't know if that helps : )