You can downgrade while iOS 11.3.1 is still being signed. You can check here: ipsw.me. After it stops being signed, youre most likely out of luck.
11.4 sep is compatible with 11.3.1
11.4.1 will not be
as for blob saving on A10/11 devices Coolstar has stated the following nonce's are required.
15400076bc4c35a7c8caefdcae5bda69c140a11bce870548f0862aac28c194cc
833e50b9c6a4fbfbdc51144a60b4cf25be3a0a4742ca2b7bd6f5ec06905443ac
d8f682df87d812c372491b613d59795a80383f439587c0bb511ccf6865eb87cc
tsssaver by conan will probably do the above automatically.
so if you saved it before try and do it again to make sure your blobs are with the above nonce's