These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.12 s (backtrack: 0/20).

[ebuild  N     ] virtual/fortran-0-r1::gentoo  USE="openmp" 0 KiB
[ebuild  N     ] sci-libs/openblas-0.3.34::gentoo  USE="openmp -cpudetection -index64 -pthread -relapack -test" 24734 KiB
[ebuild  N     ] sci-libs/lapack-3.12.1-r2::gentoo  USE="deprecated flexiblas lapacke -doc -eselect-ldso -index64 -test" 7879 KiB
[ebuild  N     ] sci-libs/flexiblas-3.5.0::gentoo  USE="openblas openmp system-blas -blis -index64 -mkl -tbb -test" 80741 KiB
[ebuild  N     ] sci-libs/blas-lapack-aux-wrapper-6::gentoo  USE="-index64 -test" 10 KiB
[ebuild  N     ] virtual/cblas-3.8-r1::gentoo  USE="flexiblas -eselect-ldso -index64" 0 KiB
[ebuild  N     ] sci-mathematics/flint-3.5.0:0/23::gentoo  USE="-doc -ntl -test" 6226 KiB
[ebuild  N     ] sci-mathematics/arb-2.23.0:0/3::gentoo  USE="-test" 1944 KiB
[blocks B      ] sci-mathematics/arb ("sci-mathematics/arb" is soft blocking sci-mathematics/flint-3.5.0)

Total: 8 packages (8 new), Size of downloads: 121530 KiB
Conflict: 1 block (1 unsatisfied)

 * Error: The above package list contains packages which cannot be
 * installed at the same time on the same system.

  (sci-mathematics/arb-2.23.0:0/3::gentoo, ebuild scheduled for merge) pulled in by
    =sci-mathematics/arb-2.23.0


For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

https://wiki.gentoo.org/wiki/Handbook:X86/Working/Portage#Blocked_packages