Suzann,
First, regarding the Charger for your Cell Phone. You'll need to check the specifications on the Charger to determine the Input Voltage. If it states "100-240 VAC", it's designed for world operation and all you'll need is a Plug Adaptor. As you'll be travelling in both the U.K. and Germany, you'll actually need two Plug Adapters - one for the U.K. (large rectangular pins) and one for Europe (two small round pins). Let us know what the spec's are on the Charger.
You might want to speak with the T-Mobile staff to get a better idea on what your costs would be for roaming. I'm assuming that the "international plan" that you mentioned has a monthly charge? You might be able to sign up for the plan and then cancel it when you get home?
As you plan on using the phone only occasionally, my preference would just be to use roaming with T-Mobile, as it will be a lot simpler. If you buy a SIM card in either the U.K. or Germany, when you travel to the other country, you'll be "roaming" and the rates will be higher. Also, your phone number changes each time you change SIM cards, so you'd have to notify those that might want to call you. Keep in mind you can also send texts to Cell phones in the U.S. and the cost for those is usually quite reasonable. I use that method quite frequently to keep in touch with family back home.
If it turns out that you do in fact need a Voltage Converter, I can provide further information on which type to buy. I'll do a bit of research and see if I can find any information on the Samsung Blast.
One final point - if the phone is capable of net browsing or E-mail, be sure to check on the rates! Data roaming can be VERY expensive!!!
(* It appears that a Travel Charger is available - ask T-Mobile about the ATADS10JBEBSTD / Price: $19.99 Charger ).
Cheers!