Is it normal for a micro SD card to have higher write speed for the first blocks?
Yes, this is normal. Many SD cards have a write buffer (think of it like a disk cache). If you read the datasheet for the specific component it should detail the size of the cache and access speeds of the device in general.

If you think that the speed is based on the block the test is easy: write to the SD card at some offset past where you think the "fast blocks" are -- they should be just as quick for the first writes.