I suspect you have the D1 (R1- the original) and you can see from the schematic http://www.wemos.cc/wiki/lib/exe/fetch. ... =en:d1.pdf which of the IO lines have 10K pull up resistors (GPIO0,GPIO2) or pull down (GPIO15). The information is also in the pin definition table http://www.wemos.cc/wiki/doku.php?id=en:d1#schematics. For the D1 R2, there is an analogous table http://www.wemos.cc/wiki/doku.php?id=en ... schematics although I don't see the schematic for that board.
As has been mentioned, those pins will act differently than straight through I/O and is likely why you see the behavior you report with the "blink" routine.
I hope those URLs are helpful.