lsblk is not compiled with util-linux Commit 031659024b3cf27bb68dc58d07519b563ef472a1 indicates the BR2_PACKAGE_UTIL_LINUX_LSBLK option was removed when upgrading to util-linux 2.28.2 since that version always built lsblk. However, in when using buildroot 2017.05.2 (which uses util-linux 2.29.2) lsblk is not built.
The "lsblk" utility is part of the "basic set", now. Ensure that you have BR2_PACKAGE_UTIL_LINUX_BINARIES selected. If this does not work then please run "make BR2_DEFCONFIG=/tmp/my-defconfig savedefconfig" and attach the resulting /tmp/my-defconfig to this bug report, so I can try to reproduce the problem here.
I just noticed that commit 031659024b3cf27bb68dc58d07519b563ef472a1 did not add the required entries in Config.in.legacy for BR2_PACKAGE_UTIL_LINUX_FINDFS and BR2_PACKAGE_UTIL_LINUX_LSBLK selecting BR2_PACKAGE_UTIL_LINUX_BINARIES.
No answer from the submitter in more than 30 days. Closing as invalid, since lsblk is enabled by the "basic set" option, as already explained.