Blue Lagoon in October - yes or no?

You get out of the water and go into a locker room to get dressed. I don’t think it is worth a stop if you don’t plan to bathe in the water.

Since it will be around 40° outside, how does it work when getting out of the water and walking to where my towel is on a hook and walking to the locker area? Is the area warm from the hot water?

Yep, still the same process -- enter through an indoor pool and swim through a door to an outdoor pool. The heat from the water keeps the air just above it warm. I've been in October and March and wasn't cold either time.

(Sorry, not POOL, obviously -- the outdoor lagoon area.)

I was just there last week...there is a swim-up door that you can go through without getting out of the water. It leads to a small pool with an indoor ramp that goes directly into the indoor area where you keep your robe and towel while you're in the lagoon. You don't have to get out of the water in the freezing cold. They really have thought of everything!

Is a hair dryer provided?

Yes! There are a number of them you can use in each locker room.

For those that have been to both, how does Blue Lagoon compare to the public geothermal pools in Reykjavik? Are there mostly tourists at Blue Lagoon?

Are there mostly tourists at Blue Lagoon?

Likely almost 100%...no one else would pay the exorbitant prices to go there, and the place is definitely marketed to tourists. Having said that, it's worth going to at least once to experience it (I went mostly for the photo opportunities since it's a really neat place). Locals use their own neighborhood pools, they don't need to trek out to the Blue Lagoon. I went to a local gym and swimming pool in the suburbs of Reykjavik and it was wonderful. Mostly locals, did not notice a any tourists (maybe there were some, but none that I noticed - I doubt they'd go out of their way to go to the 'burbs for a gym/pool).

The local pools aren't quite as atmospheric as the Blue Lagoon but they are also less than 10% of the cost. I think the Blue Lagoon is worth visiting once - it's quite a place - but in the future I'll just use the municipal pools.