It appears that the border is set by the following style definition on the block container (it is in block-editor/style.css):
.block-editor-block-list__layout .block-editor-block-list__block:not([contenteditable]):focus::after {
position: absolute;
z-index: 1;
pointer-events: none;
content: "";
top: 1px;
bottom: 1px;
left: 1px;
right: 1px;
box-shadow: 0 0 0 var(--wp-admin-border-width-focus) var(--wp-admin-theme-color);
border-radius: 1px;
outline: 2px solid transparent;
}
Changing setting box-shadow to “none” removes the border.
Thus, adding this css will remove the border:
some-selector-to-the-container:focus::after{
box-shadow: none !important;
}