I tend to get blisters on the bottom of my feet only in the summer time. What could that mean?

Possibly increased. Sweating or the types of shoes you are wearing. Some tend to wear more flip flop types of shoes......so careful with the choice. Some may go sockless as well which may be contributory.
Sounds like fungus. In summer time feet tend to perspire more, sometimes leading to athletes foot/fungus. Wear absorbant socks, change regularly and let feet air out when possible. A small amount of powder in shoes is also helpful in absorbing extra moisture. Antifungal cream or spray should also be helpful. If this is severe or persists, see Podiatrist or Dermatologist.
Blistering causes. I'm not quite sure what you mean by "blisters". Blisters can be caused by friction allergic reaction and infection. Summertime shoes are general held on by the use of friction. The friction could be the cause. Summertime shoe may has a chemical additive to the insole. This additive could be causing the blistering. The sweating feet during the summertime could make a fungal infection more possible.